With the exception of a tiny percentage of the population with extreme situations, such as soldiers in the field during election time or a few other edge cases, people can vote in person with a free voter ID.
The reason Democrats want mass scale mail-in voting is simple. Lets say half a percent of the population votes by mail. It would be too hard for them to suddenly find hundreds of thousands of mail-in ballots that voted 99% Democrat and have it not be too suspicious to defend. However, if mail-in voting becomes the norm, it becomes a LOT easier to inject hundreds of thousands of magically produced mail-in ballots. Democrats want to set up a situation where it is easier to get away with mass scale election fraud, plain and simple.
Voter-ID is a problem for Democrats. It would mean their lazy welfare voters would have to put some effort into voting, and if that's the case, they'd say to hell with it and just stay home. It's why the Democrats are pushing for mail-in across the country. The more people who won't put any effort into voting and mail-in instead, the more politically ignorant voters you get, and politically ignorant voters mostly vote Democrat. You know, the Obama money and Obama phone people. And of course, look how this past election turned out.