Regarding default ps output: remember how people were using Unix in the old days. Logged in via a text terminal on a multiuser system, 99% the only processes you care about are those associated with the current terminal, and that's what ps shows by default. You might have half a dozen backgrounded programs running in that terminal, but that's about it.

Now, my X sessions always have a dozen xterms running and plain ps isn't very useful, but it might break scripts if they changed it...