Perhaps because there's no subpixel hinting, I guess?
Haiku does have it (it was hard-disabled for a long time due to patent concerns), but there's a bug relating to how the main text control draws strings that causes characters to overlap one another with it enabled, and so things generally look worse when it is. Someone should take the time to fix that...
Previously the default font was DejaVu Sans, which really looked a lot worse.