It already exists. Part of the problem is that it only exists in direct-drive hub motors. Geared motors, which are lighter and have other design benefits, freewheel when coasting, this negates any potential for regen. Mid drives are almost all of the geared motor variety, there is also the point that using regen on a mid-drive would send chain tension the other way, most bicycles are not set up for that.
So, while it exists, it is not too common and direct-drive systems seem to be losing market share in the mid to high-end bike market. With that said, I don't expect to see much of it, some, but not much.