I find the opposite question equally as valid. Why is it assumed that a programmer can bootstrap a successful SaaS alone?
To build an application, you need someone who can identify an opportunity (product market fit.... and that's harder than most would assume). Of course even if you have something customers want, you still often need to help them want it... so you need a marketing/sales guy.
Implementation is the last piece of the puzzle. I think these are completely different skill sets, and a successful company 90% of the time needs 2 or more people to pull it off.
Just look at all those scams on kickstarter, the one thing they're missing is a viable product that an engineer can tell them is impossible.