Supervising people is a lot of work. Even if you give them their space to get things done and come up with their own ideas, there is an enormous amount of my time that goes into making sure no one careens off the road into a ditch... or at least not too deep a ditch.
While this is not limited to new students, certainly the time/benefit ratio of more advanced trainees is skewed heavily towards the benefits. The thing that never fails so make me smile is when someone comes in with that look that is part excitement and part unsure. Usually it means they have some new data, a new idea and a new direction they want to try. Nothing makes me happier than when a student or postdoc comes in and says "Our new data tell us XX and I want to take it a step further using YY to really see what is going on. I know we don't normally do YY, but I think it'll nail this down".
That is why I spend my time writing these proposals and jumping through these hoops.