However if you want to vote in person, you have to wait in lines for hours and hours and you have to actually be there and you have to maybe take that day off from work if you can even do that, so basically it takes a ton of stuff to actually do it, who would really want to do something like that when you can simply just mail your vote without having to worry about none of that. That is the reason why Trump hates it, if they allow mail voting everywhere around the USA and make it super easy, republicans would never win a single more election, there would be only centrist democrats vs progressive democrats.
On of the solutions could be electronic voting system. Some countries in the world already have it (like Estonia) and its functioning well. The chances for misuse are minimal and everything is very fast and convenient for citizens. But I guess Trump wouldn't like that way of voting either.