When voting in person, the poll worker is checking your name & address against what's listed in the poll book. Not listed? You get a provisional ballot. Two people try to vote as the same person? That'll be caught and investigated. Show up to vote as someone else (who doesn't vote)? Well, that's plausible.
Most of these kinds of "fraud" are actually voter registration "errors", where someone votes in the wrong jurisdiction. Like Ann Coulter did.
As for postal ballots, ya. The voter signature and proxy name are mostly for after the fact investigations if problems need to be resolved. You should attend your canvassing board meetings to hear all the nonsense. Husband and wives swapping envelopes. A couple putting two ballots into one envelope to save a stamp. Ballot from a prior election. Soiled ballots. Missing ballot, so voter just used another piece of paper (still legal). Etc, etc.
That spouses, job bosses, church leaders, command officers "help" others vote (the correct way) is established and well documented. But society seems to have decided that the benefits of enfranchisement outweigh the downsides.
Remarkably, comparatively, there's very little abuse of postal ballots. We're talking 10s and 100s vs 1,000s and 100,000s who are disenfranchised by other means. Caging, gerrymandering, underprovisioning voting machines and polls sites in key areas, not counting any votes on spanish language ballots, calling in a bomb scare and shutting down the central count which then has a mysterious miraculous election result, ad nauseam.
There's no shortage of problems with our districting, campaigning, elections, voting, tabulation, vendor relations, etc. If you only care to look. But please don't make the mistake that retail voter fraud is a significant factor.