When you register to vote, you give your address as well as proof of eligibility to vote. That address is used to assign you a polling place, and also as an additional piece of data needed in order to filter out fakers. Your voting eligibility is checked before being added to the list, which also mitigates fakers.
If you're trying to register in someone else's name, you have to pray that they don't register themselves or show up to the polls to vote. That's a gamble which prevents systematic individual voter fraud.