Problem is, again, it requires one of a few things
1) millions of non-citizen voters
2) millions of people travelling from area to area on voting day
3) millions of people voting for dead voters still on the registry.
For some reason, though, when people actually scour the records, they find, like, 3-4 people per state who have green cards and were registered accidentally but didn't vote, an ex-felon who was barred under some Jim Crow-era disenfranchisement law, and maybe that blond woman that used to date Bill Maher, whose name I can't be bothered to remember because she's like two models out of date, with Tomi whoever and then Gun Girl taking her place.
Point is, if there were really hundreds of thousands of these cases, they'd actually be rather easy to find many of them, just by looking at various records. And people tried, and came up empty.