In a lot of cases, the applicants/attorneys don't actually do a search, because doing a search means that if they fail to tell the Office about something they find, they could be on the hook for it later. So it's safer to be willfully blind.
As for the USPTO, we do what we can in the time given. There's only so much searching we can do, and if we can't find and present evidence that something was already known publicly, we can't just send out rejections based on how many Slashdotters think it's obvious. Issued patents aren't perfect, but when you compare the claims that get issued with what was originally filed, you'll see just how much worse the system could be if we actually did just rubber stamp everything.