Not sure if they have gotten dimmer. Possibly the opposite: I don't remember having noticed it recently (but I've also travelled a lot less), but I do remember being very annoyed by this many years ago.

No idea why hotels were doing it, neither LEDs nor the electricity for them are that expensive and I doubt anyone wants to be stuck in a dark cave.

That's probably intentional to prevent you from realizing how unsanitary the room really is.