Besides that they also lack the amount of control over the software side. Google is the entity that controls the stack, not Samsung - their responsibility ends at the kernel / HAL interface.

So why should Samsung invest more than the bare minimum when they can't get anything measurable in return?

This is, of course, totally untrue. Samsung changes a ton of things about their flavor of Android, at every level of the stack.

Edit: Samsung has apparently already ripped out the Bluetooth stack and replaced it with their own - https://news.ycombinator.com/item?id=26650447