Uh, until like 2018, they literally couldn't. Some deal between the DNC and GOP that a judge was coming out of retirement once a year just to force it to be extended forever. The judge died in 2017 which is the only reason it finally was ended.
One of Pelosi's big demands in her 2000 page ransom note for the COVID19 relief checks was to sneak said deal back in, this time as a law instead. No idea if that got pruned out or not.
It's why you'd see shit like the DNC just happening to find a trunk full of votes in certain districts and the GOP didn't take them to court over it.
If that didn't get pruned out of Pelosi's ransom note, that would explain why they're so gung-ho for Mail in Voting all of a sudden. The GOP literally wouldn't be allowed to do anything about it.