Unfortunately it is unlikely that we provide a fully fledged and documented scripting API in the short term. See the recent discussion here about some ideas:
https://esotericsoftware.com/forum/d/23924-plugins
As for why: We are a small, tightly knit group of enthusiastic professionals who have gotten together to build Spine. We don't have tens of billions of dollars like OpenAI or hundreds of billions of dollars like Adobe or other large companies. We don't have the time or budget to do everything we want, which is a very long list that includes scripting and many other things. We prioritize our efforts using a mix of what we see as providing the most value to the most users, as well as, honestly, the things we enjoy working on.
The upside is we are more agile then big companies and have fine control over our development. We also provide top notch, personal support. You are getting a response from the owner of the company on a Sunday. Good luck getting that from Adobe and friends. 😉
