Cause many non-technical people have no useful skills. There's really only two things to solve, technical and sales. I can handle the technical.
Ideas are cheap and most suck (that is, not innovative enough to really be something special). I want someone highly skilled at the process of taking something small and good, getting people to invest, and making it bigger. I can build <whatever startup> faster and cheaper and more solid than you could ever imagine, but don't have any desire in schmoozing or learning the VC game.