There are a few people who are cautious and methodical enough to specify all types in comments and keep them correct. But you need an entire team of them, because type comments which are only mostly correct would be worse than useless. And an entire team wouldn't choose a dynamically-typed language that makes those showstopper bugs more likely.

Well, if you're having major problems keeping the project's code and comments in sync, no compiler will save you.

That's exactly how the compiler saves you. A strongly typed routine signature contract is a like comment that cannot get out of sync.