I mean probably because they would break, no? I think glass-looking buttons are great (think Sony's Dualsense controller, Xbox controllers, tbh many controllers have glass-ish buttons)

I think it's a nice aesthetic. It obviously needs some tuning (contrast, transparency, etc.), but the idea is nice! I've installed the beta, and it isn't as bad as it looks, just takes some getting used to.

I also theorize this may be some grand transition phase to prepare everyone for the visionOS future apple wants to happen, but that could just be a stretch.

There are myriads of glass controls around you, just pay attention to it. From car interiors to elevator buttons, it's there.

Glas actually makes sense, given its an extension of the device's hull.

Probably the main reason is because they have ugly electronics behind them instead of pretty dynamic colors.