AssemblyScript initially targeted TS->WASM compilation, by only supporting a strict subset of the TS language. But at some point they dropped that idea and defined their own TS-like language. I don't know the reason for this, but my guess is that TS is too dynamic to just directly compile it to WASM?

Microsoft has done exactly the same on their MakeCode IoT compiler, using TypeScript and Python.

"MakeCode Languages: Blocks, Static TypeScript and Static Python"

https://makecode.com/language

It is very hard to generate good AOT code when dealing with dynamic languages, of any kind.