Button or switches need to be open or closed. A spring strong enough to force the button to switch when pushed is used because if you were to push it down and let the contacts touch slowly, the contacts would bounce and you'd get multiple clicks with a single press (undesirable). Mice tend to have very little button travel which doesn't lend itself well to using other kinds of switches which allow smoother / quieter switches.

Some quieter or less clicky switches probably do exist but its at the tradeoff of greater travel distance, mushy feeling or double clicking. I recall a mouse with rubber dome buttons that was discontinued because it feels like crap ans no one bought it