There are lots of great reasons to buy SaaS businesses. Failed product market fit, for example, does not mean that creators tried everything they could. If you're a marketer (or already have an audience) buying a product is a great idea.

Lots of people also sell good SaaS products because they have others that have taken off (I bought one last year from someone who got funding from YC on another project, for example).

"The market likes the product. In this case, the original owner can just hire people to do the work, so there's no reason to sell." is also flawed. Do you have any idea how many developers/creators hate the idea (and reality) of hiring and managing people? Plenty would prefer to cash in and go back to building and let someone else build a team.