I agree that there may be a lot of good assets to find, even free ones. However, the difficulty is to combine them in a way such that the game doesn't look like a patchwork, which is usually far worse than programmer art.
But I don't think that most programmer art sucks. If you check out the SFML games in the project forum, there are some with really nice graphics. Even if not all of them can compete with professional art, they still look good. Of course there are also many games with simple graphics made in Paint, but they tend to be smaller projects (often from beginners), where the focus lies on programming and not a shiny result. Correspondingly, the developers are less ambituous at making good graphics.
For music, it's a bit different -- I believe it is much more difficult to create something that sounds interesting and not boring or annoying. Also, it might be easier to find a music theme and some sound effects that match the game. Especially for sound effects, it's not obvious when different artists are involved in the making, while different graphic styles are well recognizable.
Anyway, that's only how I see it
