Well, my keyboard does not require me to press the key through to the bottom to register the key.
From an engineering standpoint I would be very curious as to what the advantage of using a metal spring over a plastic one is. I am not sarcastic in that. If you really wanted to you could make a plastic keyboard just as noisy as a mechanical on, I bet. It's just there is no reason to.
It has nothing to do with a metal spring compared to a plastic membrane. Or well, it does in the fact that membranes sometimes stick, but that's not the reason why you would switch. With that out of the way, you don't need to fully press in a mechanical for it to register, just as a traditional keyboard tends to send an electrical signal without fully touching the circuit. The main thing is that mechanical keyboards press in a linear fashion. Unlike a traditional keyboard, where the keys go all over the place. Mine currently wobble as if I was playing with a controllers joypad. The best part of mechanical keyboards being the variety of cherry types. The four main being two with feedback and two with a straight push. Some have different resistances to pushing in, some are quieter, while others are easier to double tab, etc etc etc. You won't fully grasp this until you try them out yourself.