I'm not sure how "easy" is to cast fraudulent mail-in ballots - there are states like Oregon that have entirely mail-in ballot and don't have meaningful issues. This is something that thoughtful people have been working on a decades now and there isn't compelling evidence that it supports fraud. The person still has to be registered and the mailed-in ballot contains verifications and attestations that are checked against the rolls. But I don't disagree with you that internet voting is a whole pandora's box that we're just not ready for.
The biggest problem with exclusively in-person voting is that (1) some people have disabilities that keep them from being able to get there, and (2) some people just aren't around where their permanent residence is because they're away on military, service, or work duties. Some states have requirements that you prove the need for an absentee ballot and I'm okay with that - but also other states have what are demonstrably successful mail-in ballot programs.