Different projects use different versions of typescript with different configurations, and almost no one wants the client to throw type errors where the untyped code would have actually worked. I.e. the are drawbacks but no advantages. This is ongoing standardisation work to allow it to _ignore_ typescript and just run the JavaScript which will be nice but not a huge win.

It's not a stupid question, but here is the TC39 proposal to add type declarations to JS: https://github.com/tc39/proposal-type-annotations

I'm not enough of an expert in PL theory and compilers to say for sure, but I suspect that because Typescript's syntax and semantics are all designed with total type erasure in mind, there's no advantage you could get to executing it natively versus compiling to JavaScript and taking advantage of all the optimizations JS engines already have.