Because you need to both know the value of an action (ie, which politician the vote is counted for) and you need to hide who did the action (to keep the ballot private) and you need to ensure every voter only does the action at most once and you need to ensure that if the machine is replaced or subverted physically that the vote can't be silently switched.

No matter how you dice it, one of those things gives with electronic voting, even if you had electronic voting machines with no state (all pure circuits, say), but especially with votes on machines like personal computers, where a myriad of systems need to be trusted for the vote to register.

It isn't worth it. Paper ballots are intelligible to everyone, and even when we vote by mail there is such a paper trail it is hard to fake.

How do you verify the output has any relationship whatsoever to what voters input in it?