If people were voting under other people's names in significant numbers, it'd be obvious when the real voters came in and found out they were already marked. That very seldom happens.

The reason people don't do it is probably that an extra vote or two is unlikely to make any difference, so the risk/reward is poor. It's hard enough getting people to take the time to vote in the first place, much less risk a felony conviction to vote more than once.

IIRC a number of countries use an indelible ink pad that voters have to place a finger on.

"BUT ITS NOT A HUGE PROBLEM"

it still happens, which is a problem none-the-less.