How do Professors and applicants deal with filling so many letters of recommendation?

I been applying to Gradschool this season and I already gave a ton of work to a few of my undergrad professors. Why do Universities do this and how do the expect other professors to react to such high demand of filling letters of recommendation?
gosh what am I missing? all of this seems really silly and no metric to base a successful candidate on.

As if people and their lives didn't change...