Indeed, I've witnessed four aircraft land wheels up for no particular reason, and one was during an MEIR lesson! So that's two CPL holders who both managed to forget the wheels, and ignore the buzzer.
The result was that insurance became prohibitively expensive and now no one rents retracts.
Hit the nail on the head there.
the FAA or CAA would take action pretty quickly if they felt retractable gear were such a terrible risk.
What exactly do you expect them to do? There's already the three green lights, and a buzzer that sounds a low power. You can train as much as you like, but private pilots will always forget one check or two, and why wouldn't the gear end up being one of them eventually? Certainly as a low hour PPL I've landed flapless by mistake in a PA38, and only after thought, "yeah, that approach did feel a bit weird."
Perhaps we should adopt the RAF way of stating "three greens" in the ATC call when cleared to land.