I think the goal is reducing risk to something manageable for a single person without capital. If your customers don't believe in the product then you'll have to be the one who believes in the product and for a single person that is a pretty good signal to look for something more worthwhile.

And yet people do (kickstarter, for example, or those books where there is only 1 chapter done, but you pay almost full price anyway and wait for the rest)

I think there's a big difference between the types of things that succeed as a kickstarter and yet another SaaS product in a crowded niche.

Can confirm that I'm much more likely to spend money on someone's interesting creative story than on literally any SaaS