As for why: I have no idea! Maybe just for user familiarity reasons, since a lot of people grew up with that kind of analog feedback that the antenna wasn’t getting a signal.
This is exactly why. Preventing screen burn-in may be a tiny peripheral reason also, but providing a familiar experience to chronically myopic and cranky users (i.e. boomers) is probably the bigger one.
I would have left a thumbtack on every chair, and a bucket of whitewash balanced over every door.