In a hilarious coincidence, I came upon this topic just when my uni's Sci-fi and fantasy club was having difficulties finding enough DMs to organise a one-shot game night through the club chat. I guess the problem is local and global!
Jokes aside, I don't know if there's more of a shortage compared to earlier years of the hobby, but DMs are certainly a rare commodity. I had to start playing by DMing because nobody else around me was willing to run a game, and out of 10-15 people I've ran games for over the last 5 years, I'd say only 5-6 of them ended up DMing regularly. Some of my players want to dip their toes into DMing and run oneshots and maybe even short campaigns, but the task of running a long campaign that goes over 5-6+ levels seems too daunting for most people I know.
As for the reasons why that might be, the scene's explosive growth is probably the biggest reason. Far more people play D&D (or other TTRPGs) than they did at any point in history, and there probably isn't enough DMs for all of them. But I'd say 5E in particular hasn't done any favours to make it easier to raise new DMs. A lot of the advice in DMG is fairly shallow, and I end up relying a lot on stuff I learnt back in 3.5/PF (seriously, the difference in depth between the 3.5 DMG and the 5E DMG is enormous), as well as the advice of older DMs that I see on the Internet (Matt Colville, the Alexandrian, Courtney Campbell... sigh even the Angry GM). If you don't know that there are a lot of unspoken assumptions that older DMs knew but the game never mentions (for instance, the procedure of dungeon delving, which the game never explicitly mentions, and the Alexandrian makes a good point that this causes a generational knowledge gap for new GMs), you end up floundering a lot, and that discourages a lot of people. I hope that the OneD&D DMG will open up some space for these unspoken GM procedures, but I'm not counting on it. If anything, 5E's approach to DM-facing content seems to have become "Here are some interesting 2-3 sentence prompts, we'll give you no guidance or constraints on what to with them, go nuts".
Honestly, there might also be a generational gap. I know that in 3.5/Pathfinder days as well as the early days of 5E, players would thoroughly read the rules and acquaint themselves with most of the sourcebooks. But now, most of my players don't even read the full content of their own race and class, let alone the rest of the book like combat rules, equipment and downtime. It may just be that the way people consume content has changed, and people prefer watching videos about how D&D works than read books and posts about it, but it is sometimes frustrating (even though I love my players to bits).