Because it is not needed. Note I said where the switching current is below the makers wetting current spec. I did not say everywhere all the time.
If what you said was commonplace, millions and million and millions of mouse users would be complaining about their mouse switches failing all the time. This is particularly true since the vast majority of those mouse users buy cheap mice. And yet, complaints are not widespread. In fact, these mice are still working just fine after years of use.
Are there exceptions? Of course. Until Man can create perfection 100% of the time, there will always be samples that fail prematurely. But exceptions don't make the rule! So stop pretending they do.
Come on dude! Don't be naïve.
READ THE DATASHEET again. Only this time,
take the time to understand it. Your screen shot just illustrates you don't understand the issue - at all! And/or you are just trying to obfuscate the issue with out of context data so you don't have to admit you don't understand.
Do you not understand what a micro-load circuit is? Do you not understand that mice operate on volts! Typically 3V from 2 AA batteries or 5V from the USB port? These are not micro-load circuits.
Read your "out of context" screen shot again and try this time to understand what it is saying.
It is saying, "IF" you use a model designed for ordinary loads in a micro-load circuit, it "may" result in faulty contact.
In other words, what that is saying is,
"if you are stupid and use the wrong switch for the job, the switch might fail." No duh!
READ THE DATASHEET again and this time, take a moment
to see in the same D2F datasheet you referenced lists
over 120 D2F model switches with different current capacities that the designers can choose from to ensure they match the expected loads!
What that is telling the designers is to use common sense and select the appropriate D2F model designed for the expected load. And guess what? They do! And that is why this is not a widespread common problem and why they do not need to use those caps.