Because the hard part of running it is making it highly available despite the inherent fallibility of distributed computing, which is an infrastructure problem, not an AI coding problem.

Temporal is fully open source, you can run the entire thing on your laptop or your own server. People pay for the SaaS anyway, because dealing with the distributed computing problems is hard.

(There are some other projects inspired by Temporal as well, but if you want to run something in-house, you're probably funding a minimum of 2-3 engineers to handle on-call, and for that price the SaaS is competitive).