The actual difference between a working new mouse and a failing double click mouse is in the button itself (mechanical parts are almost always the problem).

However, it is not some exotic failure mode. All mechanical switches have a "bounce", where the contact makes and breaks a few times before settling into the connected position. Switches are typically designed to make the actual contact spring loaded (which is the origin of the click sound you here). As they age, this mechanism degrades, making the bouncing problem worse.

However, this is a well understood problem that any electrical engineer should be familiar with. One solution is to install a filter capacitor. Now it takes longer to switch between the on and off state, so the inherent bounce in the switch is smoothed out to the point where you cannot detect it.

They probably did testing with a new switch, and decided that they didn't need to include any explicit debounce component, ignoring the fact that the switch would degrade over its lifetime.