The burden of playing is far less than the burden of DMing (putting aside the monetary commitment we seem to take for granted hits a DM harder as the DM seems to be expected to buy more stuff, let's consider a DM that is in a group where everyone is sharing a single PHB, DMG, and MM).
Playing is far less work than DMing. Your prep maybe consists perhaps of thinking about what cool new stuff you want for your character at level up (which you don't do every session anyway), and if your DM is lucky, familiarizing yourself with your own character’s abilities, gear, etc. For beginning players especially, this kind of prep is fun because your character is often your avatar in the world and it is cool to see yourself more powerful. You don’t have to worry about balance, prepping stuff you might not use, etc. in other words, any work you do is usually about making things fun for you. As a player, you're only worried about one person - yourself.
As a DM, your weekly prep consists of: challenges you will be throwing at the players such as monsters, traps, puzzles, social encounters, etc. a good number of which may be reasonably expected to go undiscovered and unused (i.e., "wasted"), some worldbuilding of places characters might choose to go, probably some plotbuilding, creating NPCs of all stripes (important to unimportant) to support that, and by the way you are expected to be familiar with all of the rules for all of these things (this is an unrealistic expectation but so it goes) and probably know all the rules for the players' abilities as well (because you're the "judge" - you have to know the rules, they don't!). In addition, you are expected to be so familiar with doing all of this that you can improvise stuff off the cuff whenever the players do something you didn’t account for (in other words, every session). You have no avatar in the game four which you can create cool stuff (all the baddies you create cool stuff for die in a couple of rounds of combat anyway). You must be a compelling storyteller and should be a wordsmith that can evoke a feeling of realism mostly by verbally describing things in an intricate and compelling way. You are probably expected to mediate disputes between players (not just in-game disputes between characters). And above all this, it's your job to make sure everyone else is having fun and even if you had fun, if nobody else did, you failed. Oh, and in all of this you shouldn't ever let things get adversarial. As a DM, you're worried about every single person in the group, but you're supposed to worry about yourself least of all.
It takes a special kind of person to be able to attempt DMing, much less enjoy it (though it is probably good preparation for parenting) or do it well. I think the expectations of the DM were far less when I started DMing ("railroading" was much more tolerated, and so there wasn't the risk of preparing content that would go unused, the rules were much simpler and so there was far less to master, there was less expectation of plot and world building so you didn't need to spend as much time doing that). Many of the skills a DM needs take experience to master, and there is simply no shortcut to experience... and it's something a group of 5 new players simply CANNOT have.
Whether or not Mercer effect is a thing, let's be honest... first of all, there has been more time for DMs to become experienced than there was, say, thirty years ago (an "experienced DM" now may have been running for 40 or 50 years, while even the most "experienced" DM thirty years ago would have half that experience) but experienced DMs have also become
more visible - when I started running games, the only thing people had to compare my DMing to was the two or three other guys in the area running games, but now I can be compared to the likes of <insert celebrity DM here>.
The expectations on a DM have moved from "entry level" to "several years of experience" - meaning of course it's almost impossible to expand the pool of existing DMs... because you can't get the job if you don't already have the experience. I find it completely believable that a group of players has a hard time finding a DM because these days the expected level of experience of a DM is a level of experience that new players simply don't have. I'm not sure how to get people to dial their expectations back so this problem can be solved. On the other hand, I guess, job security for me as a DM.
