It's the same reason people use normalize.css rather than doing that all themselves: it's necessary, but unnecessary for you to do yourself if there's a template their for you to use.

My objection is not against frameworks or utility files like normalize.css.

The most "foundation" I have whenever I design a website is having a full-width header seldom. I can do that in a couple of minutes. Everything I design, web-wise, is unique in purpose and, subsequently, feel. It's one of the things that guide my outlook on these things. I never design for the public specifically; I only design what I see fit for the situation.

You want to design websites professionally, or for public use? You're probably better off using a popular framework as your foundation. HR will expect you to have experience with "common frameworks" like Bootstrap when they check your CV. I'm not saying getting proficient with one of those is bad or dispassionate.

My objection with Bootstrap et al. is that they're opinionated to a degree I find uncomfortable working with or, indeed, seeing around. Bootstrap, in particular, pushes a sterile, bland design doctrine with its default values, and most people using it won't bother or aren't capable of tweaking it to their preferences beyond changing color values, perhaps.

It's one of those things that allow extensive public access to a field that used to be dominated by professionals. Fifty years ago, photography used to be a hobby of someone passionate enough to spend a lot of time mastering it. Nowadays, even a three-year-old could take daddy's smartphone and start their artistic career on Instagram (even if by accident). Opinionated CSS frameworks allow people to engage their user with an at-least-somewhat-meaningful design, as opposed to having no design at all.

I think it's good that more people are getting involved in the process of web design. Most of them will never become professionals or even amateurs, but now they can at least try their hand at it with a guidance that would ensure they will most likely not suck. However, as an amateur designer, I find this kind of frameworks to be less of a help to another amateur designer and more of a hinderance.

It could be that I find it particularly difficult making sense of someone else's systems and code and would rather make my own. I'm scared into a corner by the massive amount of stuff I have to learn for any single decent framework, JS or CSS. I'd rather start from scratch and know why I'm getting the shit that I'm getting. This is a minor but important factor, as well.