because Other People will fail to type things that really ought to be.
but (1) for wide adoption, it is probably a better approach than strict static typing even with inference. (and not all inference dx are equal.)
and (b) escape opt-out can sometimes be necessary, especially when the types get at all complicated and errors abound and syntax is heck and throw up one's hands and drop the types there just in order to make progress. because the type system is bad dx ie no interactive debugger. (sure llms can help save us now, at least.)
i'm begrudgingly a typescript fan. worst excepting all others kind of a thing.