PHEVs will run their gas engines after they have not been used in a while in order to keep the gas in them from going stale, it seems odd that the braking system on EVs doesn't do the same for the brakes
If the user just uses regen braking, they are never hitting their brake when they are moving. You would have to work it somehow into the regen, but the stopping curves are completely different for the two, so you would probably wind up surprising the driver if you decided to arbitrarily engage the brake rather than use regen.
Your assumption is that regen and braking have the same behavior and are interchangeable, but I don't think that's the case beyond both of them being used to stop.
Unfortunately, you can't generate enough friction to do this without the car moving, which is why it has to be done manually.