Amen. However, the physical improbabilities you point out make me take the thought above further, to a no less disconcerting place... Given the difficulty in leaving a physical clue at the appropriate time and place, I then start to entertain the even more humancentric, but more reasonable(?) possibility that it was dropped in by development.
Ever read The Big Front Yard by Clifford Simak? Basic premise is a Yankee trader and handyman ends up housing a stranded crew of rat-sized aliens who use his handyman horde to open a wormhole in his basement so they can get back home (they convert a TV he's working on to color as payment). He hops a scooter and checks out the environment and engages the Earth in interstellar trade because apparently we're the only civilization thus far to come up with colored paint. There was another story in Analog back in the late '80s, whose author and title I have long since forgotten, that posited aliens have been among us for decades or centuries and primarily hang around to export intellectual property. The hypothesis put forth is that most any number of planets will come up with a "rabbit" as there are only so many stellar ecologies, which means there are only so many metabolisms possible, which means there's only a limited range of paradigms for life because hell, we see plenty of examples of parallel evolution on this planet why presume any other would be different. On the other hand, there's only one "wascally wabbit." The circumstances and coincidences that led to the creation of Bugs Bunny make him a much more valuable commodity than "rabbit" or, certainly, "water." Scientific American had an exobiologist go crazy one time and she basically argued that there's like four or five predominant "plant" metabolisms, of which chlorophyll is one, so you could bloody well look for life by looking for a handful of chemical signatures and skip all the uncertainty. Combine that with the fact that if you can skip between stellar systems with any regularity? Your resource issues are fuckin' solved, man. The amount of stuff in the universe nobody cares about compared to the amount of stuff people are living on? We could have reavers from Alpha Centauri strip-mining the asteroid belt and could we really give a shit? Especially if they gave us a copy of Cold Fusion for Dummies and said "go away kid ya bother me?" I'm left with the fundamental belief that getting from star to star is a real bitch and takes a really long time but we're a lot more likely to see a probe than we are to see alien soap operas.