Because, a high-dex 1st level character can easily have a +8 modifier on the roll, so hitting a DC 20 is a decent shot, but not an outright given, for a generic 1st level character of the sort you'd expect to be picking locks.
It is a genre thing, really: Starting characters have a shot at dealing with simple, basic stuff, but it isn't handed to them on a silver platter.
Actually, given the Take 20 rule, there is no way a 1st level character cannot unlock a DC 20 lock.
What? How is it anymore dictated by the game rules than 4e's Heroic Lock, Paragon Lock or Epic Lock...
You can't have it both ways though. In 3e, locks are dictated by the mechanics. In 4e locks are dictated by the in game situation. A lock is as complicated as the DM deems it needs to be to make the game interesting, instead of the mechanics telling the DM that the lock must be a particular DC.
If I want the lock to be very difficult, I can, regardless of how skilled the PC's are. In 3e, the lock is dictated by the mechanics and I have to over rule the mechanics (something I certainly can do) in order to make the lock fit the setting.
Thus, locked doors remain in play regardless of the PC's level, whereas in 3e, locks as written stop being challenges by a certain level. And that level isn't all that high with the Take 20 rule in play. A by the book, most difficult lock, pre-epic, only needs a character with a +21 skill check to be automatically bypassable. +21 skill check is reachable by about 10th level without too much difficulty. So, in 3e, locks become redundant about 10th level.
All dictated by the mechanics.
See, the problem I have with this is people seem to want it both ways. If 4e divorces the mechanics from the narrative, and 3e doesn't, that means that 3e mechanics dictate the narrative. They have to. You cannot link the mechanics to the narrative without having the mechanics dictate the narrative.
Now, divorcing the mechanics from the narrative has problems - CaGI is a good example, but, OTOH, it has some advantages too - greater flexibility. OTOH, wedding mechanics to narrative has some advantages - greater consistency, and some disadvantages, less flexibility.