If you use the default AWS managed key rather than a CMK, it's largely the same levels of protection and the main difference is that you don't control the rotation schedule of the top-level key.

And yep, a CMK can span many services and still give you a single point of control.

But compliance is still the major thing. Many regulators, auditors and compliance authorities are just happier with KMS being the point of control. Even though AWS operates KMS, and the services, KMS is a neat compartmentalized hermetic system with HSMs at its root. So it's quicker and neater to evaluate its surface area, TCB, and so on and it's a familiar pattern to finance and health regulators especially.