I think most people are unsure how to make the transition from having a good idea to forming a company around it. Many people here build stuff all the time but often times don't seem to believe in the idea enough to put money into it, yet also are unsure how to get money from others. My guess is that people just aren't sure how, in spite of PG's best efforts.

My other guess would be that many ideas are largely just modifications of things that already exist that a large company like facebook could easily replicate. Coming up with brand new ideas is not easy.