In many cases, they (understandably) don't reply (or they are uncomfortable with sharing them, as they often contain unpublished material). Also, I do this often, and don't want to be known as "the person who asks people for their powerpoint slides".
At the same time though, it's simply far easier to remember the content of a presentation/talk when you actually have access to the stuff inside (and I do discover that I learn faster from talks than from any other source). Most of the time, I request the slides as a reference for learning the material (since I'm still new to the area, and they are quite helpful for that).
(though I do wonder - what are the underlying circumstances when most people ask for them?)