Not to mention, if everyone has to travel to vote, you're mostly going to get retirees and people with a lot of free time voting.
It is important that people are able to vote without being controlled by their family members or employers, so any unconstrained remote voting should be an exception that's minimized as much as possible.
The issues are:
1. young people don't vote because they don't care, so it should help to make it easier for them.
2. once you're in the voting booth, it's too hard to remember who you decided to vote for when there's tens of choices and ballot props, like in California.
Mail-in voting also helps prevent the issue of local governments trying to sneak stuff past the voters by having offseason elections for it. Machine politics has died off in most places, but it's still strong in New York for instance. If everyone gets a mail ballot they'll notice.