So add a hundred bucks to the price.
As for generics, when you're building a low end low power PC they're great. If you think your comp is going to draw 350w tops then there's hardly a need for a great PSU. I do agree about mid range and higher PC's. Generic parts are a bad idea on a $1k rig.
As for why the OS, case, CD & dvd etc are left off
PC's can be assembled and old crap ported over, mac boys usually don't get that luxury. I've used the same copy of windows XP since it was released and been through 3 systems since then.