What libraries are you using? I haven't found anything that seems to match the easy OpenAPI schema generation and validation that FastAPI and Pydantic provide.
I write my own validation logic and use the same code on the frontend (React+MobX) and backend. Automatic API docs are less important for me as I don't have third party API access and use TypeScript interfaces as API contracts. I run my own business and stuff like validation is something I write only once, and perhaps update and extend every now and then. If you develop many projects for external clients, this setup might be to involved and I can see why something like FastAPI is a timesaver.