I don't think many people who want to do this have the ability to. Distributed computing is an amazing thing, and many people would love to do it as far as Bitcoin mining is concerned, but the development process of it is a little tricky.

Essentially you have to write the platform for multiple architectures (x86, x64, i686, ARM, etc.) so that you have the ability to work across many systems, and that's not including writing the binaries for multiple Operating Systems. Then you have to develop a client-server model that specifies how to distribute the work and report back the results, etc., and that just covers the basics of what has to be done.

It takes some work. I wouldn't even bother with a task like that.