Many don't because they don't have products to sell. That, and many programmers just aren't that entrepreneurial.
The ones that are entrepreneurial are rich. Think Bill Gates and Mark Zuckerberg. Think Jeff Atwood and Joel Spolsky. (Those last two have pulled a coup like none I've ever witnessed.)
Not every profession has this kind of mindset. I sense it's a contributing factor to the fact that the Personal Finance and Money Stack Exchange has been in public beta for over 1,000 days. There just aren't that many people knowledgeable about that subject who see the benefit to their careers, or their bottom line, by building someone else's web empire. They build their own empires.
Some do it for the same reason they do it here -- because they like to help. But there's a miniscule direct payback for their work. The top-ranking user over there has gotten less than one profile view for each question he's answered.
If people want "conversions" for a product, there are far better places to do that than their profile page on the SE network.