It's not that they can't (mostly), it's that it's much harder.

TypeScript gives you a lot of strict typing for free. You can just throw data around, and TypeScript keeps track of its type. If you paste a blob of JSON into TypeScript, there's a new type created behind the scenes, and you can even reuse it with very little extra code.

The magic parts are the structural typing (overlaps quite a bit with duck typing) and type inference. It means that even your basic JavaScript with zero type annotations are type-checked against each other. If you just feed JavaScript into TypeScript, you'll uncover a lot of errors you didn't know were there.