Because people don't do a search for their question. They just ask what they want .
It depends on how the question is constructed. If its plain facts then its probably because they didnt use the search box. If its more of an opinion seeking or personal question question its probably due to that.
Beyond that.... Boredom I suppose. Or testing the waters.
Repetitive derp syndrome is a serious condition though. It can cause many duplicated questions! D:
If it keeps happening it just becomes spam and the moderators kick them out, of course.