IME unless you are "in a gamer setting" (hobby shops, campuses, etc.), finding players has been more difficult. As older adults, I've lost players mostly due to conflicts such as family, work, moving, etc. At this point, my Saturday "group" is almost to the point we hang out, talk shop, etc. almost more than we play. One of the guys in this group also DMs as he wanted to try it and I've given him help with that. He's working out pretty well.
I was lucky to find my Tuesday group due to a younger co-worker that plays. He introduced me to their group, who had a DM, but for the last few months I've been DMing so the other DM can play and see some first-hand DMing from a much more experienced DM.
Anyway, as for a DM shortage, reasons why newer players are reluctant to pick up the DM mantle are:
- they lack confidence / don't want to appear foolish
- they don't want the responsibility
- they don't have the time to commit to prepping
- they are intimidated by videos they watched / stories they heard
- they have more fun playing
- they feel they don't have the imagination
- they don't believe they can handle the story-telling aspect
- and so on...
Having a mentor can certainly help, especially if they are willing to co-DM with the new DM. This is what I will typically do for a few sessions when a new DM is ready to run a game. I'll help with tracking initiative, running monsters, etc. and remind them of things they forget about or overlook.
Frankly, I would love to have a job as a full-time DM, but I am not a big person for online gaming and I've never found players around here who want/ need a DM badly enough to pay for one.