Let’s start with wives of conservative, domineering patriarch types. What chance does Edith Bunker, or the wife of a conservative Muslim man, have of voting her conscience without her husband overseeing it? If there are adult children living at home, they are going to be expected to sit down with Pops at the kitchen table as well so he can “help” everyone get their ballot filled out “correctly”. And this is something you can’t investigate or prosecute. The patriarch in question probably doesn’t even think he’s being coercive or doing anything wrong, and the other people in the household might not really think about it either. But if they are required to go into a private voting booth to vote, they can vote however they like and he’s none the wiser. (For this reason, I also don’t love the common setup you see that’s about as private as a urinal: I prefer toilet-stall level privacy, like those classic booths where there’s a curtain that doesn’t open until you’re done voting.)
Churches and employers can also host gatherings where they “help” parishioners or employees fill out their ballots. As long as people are technically free to refuse to do this, there’s no crime being committed. But there is subtle social pressure. Once again, do you want to tell your boss or minister (and the peers in your “church family”) that you don’t want their “help”? That is going to make them pretty suspicious of you.
And of course you can literally sell a mail ballot. You could try to bribe people to vote the way you want with a secret ballot, but you can’t control what they will actually do in the voting booth.
FWIW, this is a position I have held for many years, way before Trump slimed all over it. (My heart sank when I saw this happening, primarily because of the clear and present danger to our democracy, but secondarily because I knew it would raise the popularity of vote-by-mail among Democrats more broadly, just to be reflexively anti-Trump.) Here’s an example from four years ago, but it was already a longstanding pet peeve of mine: