The one reason that comes to mind is that doing it implicitly lets you use structures in terms of interfaces even when those structures come from codebases you don't control .

Suppose Bob's codebase has a structure implA that implements interface A, and you want to use in terms of its interface. If the language requires Bob to declare implA as an implementation of A, you have to persuade him to do so in his codebase. But if implicit implementation is enough, then you can just use implA directly as an A with no changes by Bob.

But yes, this is a sore point. It would be convenient to be able to declare structures as implementations of particular interfaces, even if the current implicit behavior were preserved.