If you show up to the polling place, you need to list the name and address of a registered voter in that district. How do you know this information?
If you use a relative or acquaintance whose name and address you know they're registered at, when they show up to vote it will be noted that they have already voted. They can then put in their preliminary ballot, and presumably their signature will more closely match the fraudulent one and the real one will be counted.
There are enough basic hurdles to this that I don't see how it can even be done at scale.
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.