Re: Why?
I used the original Natural keyboard for 7 years, none of the key caps fell off.
I replaced them with Natural Multimedia keyboards in the early 2000s and threw them out last year, all keycaps were still intact, even if a few were worn smooth.
I also have 2 Natural Ergonomic 3000s, no problems on either. Same for my Surface Ergonomic and the Sculpt Ergonomic keyboard. I recently upgraded that to the current Ergonomic keyboard released at the end of last year.
All have between hundreds and thousands of hours of typing behind them and none of them have lost key caps.