As someone who has been a player in paid online games in the past, I can say that the economics IRL doesn't often work out as well is it would look on paper. There are several reasons for this.
1) Scheduling: on weekdays people are realistically only going to want to play in the evenings. So unless you have a multi-continent client base (and some Pro DMs do) you are probably only going to get one session per day on weekdays, MAYBE able to double up on weekends. So you are unlikely to see a 40 (paid) hour week.
2) Group size: you are usually getting paid per person, so you want a group of 6 or so to maximize your pay. But getting - and keeping - a full group is challenging. Paid players know that they can find another paid game easily so are much more willing to drop (often ghost) then they would be in a free game. And in addition to all the normal reasons people drop games, when people's finances get tight the paid games are usually the first expense that gets cut. When you lose players mid-campaign it is hard to replace people - people who are going to pay for a game have choices, and they are probably going to want to start one fresh instead of jumping in the middle of one.
3) Competition: lots of people are doing this, from college kids looking to make a couple extra bucks to companies trying to pimp out "gig" DMs. The barrier to entry is posting some games on Roll20 or similar site. When a new WotC adventure drops there are dozens of paid games starting up on Roll20 to run that adventure. So good luck filling up 6 seats for five or so games a week when you are starting out without an established client base.
Ultimately, yeah if your are a skilled DM and willing to treat DM'ing as a job you can eventually develop a loyal player base and make a reasonable income, but it takes time to build that up, and even then losing a couple of games in a short time period is going to be a big hit to your income that you may not be able to replace for some time.