For the same reason that people depend on TypeScript.
The JS output created by Fable is fine to read, and I very rarely, if ever, need to look at it.
Note I advocated to use F# but not Fable. You're assuming in your answer that you need to interoperate with JS, why?
Because JS is pervasive.
What are you thinking of when you say "something goes wrong"?
When you use better tooling like vite/snowpack the code produced by fable is served as is, and it's really close to the F# sources, the webpack bundle used in development is what looks horrible in reality
My point was not comparing Fable vs plain JavaScript, but F# vs Fable.