I don't know that it's that unpopular. I disagree with the 100% fool proof and verifiable claim though. I can think of lots of interesting ways of screwing with it, e.x. manipulating the chain of custody or tampering with the ballots to make errors likely.
Everything is a balance. Paper ballots aren't magically 100% secure, and in person voting comes with trade offs. Ex making it difficult for the military to vote, and making it such a big production for my highly disabled sister that she probably wouldn't bother.
I happen to think the pros aren't worth the cons and that most of the handwringing around mail in ballots and machines are FUD rather than measured concerns. That doesn't mean I disagree that in person isn't "more" secure.