First, my default fonts look great because I chose them and my preferences are superior to everyone else's.

Second, I use Debian and I have a fonts.conf that's hundreds of lines long, including dozens of lines of hinting settings because I prefer that some fonts use hintfull and others use hintslight. I don't have hinting settings for every font that exists, only fonts I installed, so when a site uses a web font, there is no dedicated hinting setting for it in my fonts.conf, and so it almost always looks worse than my default font.