I've noticed recently how often-present is the idea of magic not requiring an origin or a source.
Your typical fantasy never goes into details of how it works, either: it just does, and all we have to do is think about it, wave a targeting device or chant an arcane word. How audacious of us to assume that a few individuals can wield powers of theoretically-limitless might with not much - or nothing - but their minds while the rest ought to do no more than marvel and awe at the thing shining bright in unnatural colors.
To me, it doesn't make sense. Saying that magic just works is like saying that hammer just propagates the nail further into the medium, despite the fact that you are actually required to produce certain motions with no less than the necessary amount of strength for it to happen - and that's not taking into scope the idea that the nail might be deep underwater, which will require even greater strength application, and breaking the water tension if the tool is above the surface, and keeping the pressure on the hammer constant so that it won't stop due to drastically increased pressure and density...
You get the point. Many magical systems are as simplistic, which can be a good thing if your intention is to tell the story rather than tell about the world, or if the former dominates the latter. I can't say, however, that fleshed-out systems aren't necessary or aren't worthy of the time and effort; they do, however, appear to be a work of a higher skill level of worldbuilding - which I suspect to be the main reason for most of the fantastical works not to include it.
And - alright, suppose we've described its origin, be it a deity, an other entity (like the Weave in Faerun - I think that's its name), a completely different plane of reality (or outside of it) that translates informational chaos which allows you to bend the matter and energy inside for a bit of time, a law of nature (or, indistinguishable from it, a synthetic tool so natural to the world by the time the people we watch arrive that they don't see it as any foreign)... Describing how it works - precisely, how it flows from its origin into the reality - becomes a difficult matter.
Handwaving it by saying that our mind is powerful enough to "transfer" the magical energy is the most common method. Some put their money on rituals, which is saying "I beg my god to do my bidding" - again, incredibly audacious of us. Some allow their magic wands (whatever those might be) to do their bidding, instead, by making it excrete some sort of magical energy for the same purpose. That's a lot of energy, isn't it? It never seems to run out. Why didn't we appropriate it for some powerful engines to power the lightbulbs of the world forever?
So, what can we do about it? Is there a way to make magic sound plausible without breaking immersion or suspension of disbelief? In other words - can we describe some sort of immensely-powerful reality warping that would still make sense to us who don't live in the fictional world?
"Sensible magic" is human-dependant and is subject to change with our perception of the applied sciences in our lives. Superman can fly? humans can also fly with the right tools. Superman can hear things from miles away? humans can also hear things from miles away. Superman can set matter ablaze by eminating x-rays from his eye blobs? at some point we might all have our own little x-ray gadget. I find your point about fiction not often explaining the mechanics involved in its plot to be mostly valid, however -- and linking it back to our reality -- when new technology arrives to the end consumers it is very seldom described minutely and would tend to go as far as to say "It uses X technology" and be done with it. There is disconnection between the processes of technology advancements and the crowd, with probably the patent industry to be blamed for it. We probably could, but the extent of how interesting it would end up be is also a crucial selling point in books and films. I only know about a handful of documentary films that have been able to capture my attention with their description of the workings of reality impacting, so-called "magical" innovations (again, related to our time in the history and the way we define magic). Revolution OS, Linux/Software history documentary film Into the Future 01 - Transport, describes battery and electric charging ideas Just off the top of my head. And though I present a small data point of published media describing technology, I find myself able to logically extrapolate the (low) popularity of these as opposed to some other, non-descriptive films showcasing magic without presenting the mechanics behind it or why it should logically work (Many cartoons in popular media, and big-budget movies in the cinema). Can we describe some sort of immensely-powerful reality warping that would still make sense to us who don't live in the fictional world?
So, if you've decided to describe a magic system in details, the point becomes to balance the interesting with the overall. I feel like even if a lot of what I tell about the system is fascinating, the reader will notice at some point how wide the information stream is and how exhausting it might be to dive into.And though I present a small data point of published media describing technology, I find myself able to logically extrapolate the (low) popularity of these as opposed to some other, non-descriptive films showcasing magic without presenting the mechanics behind it or why it should logically work
I think ultimately a problem arises when trying to delve too deeply into a magic system, to explain its every quirk and function. I loved reading Robert Jordan's Wheel of TIme series and his 'magic' system is incredibly complex - one of the most complex I've ever seen. But he could often get very bogged down in explaining away every little facet of every little 'spell'. It could become tiring. Brandon Sanderson suffers from similar problems, I think. His magic systems are incredibly unique, unlike any I've ever seen, but he spends a lot of time explaining. Every. Single. Process. He's a brilliant writer, and his Way of Kings series is one of the best fantasy series I have ever read. But even from his opening in the first book, it is clear that he suffers from the same trap that I think all fantasy writers fall into: we want to show the reader every little detail that we can. We get lost in the myth of our world-building, and sometimes that can really get in the way of keeping the narrative going forth. I don't really mind 'magic' or any of its equivalents not necessarily having a proper source that 'makes sense'. I've been reading a lot of myths and a lot of folklore lately and to be honest it's really refreshing to just have the stories take the existence of magic and of the supernatural as just as a matter-of-fact reality of life. Edit: I forgot to mention. Brandon Sanderson also has a fantastic rule-set to bear in mind when developing a magic system
Some things can be said in passing, leaving the readers to figure it out, without spending paragraphs describing it. The mastery is to be able to locate those things and tell about them in most laconic manner possible. A far less skilled example would be saying "her luscious green hair" instead of entitling an entire sentence to "Her hair was green and very attractive". Doing this too often will come off like a subtle infodump still, but between the two evils it's the lesser.We get lost in the myth of our world-building, and sometimes that can really get in the way of keeping the narrative going forth.