There really isn't a "Tank Shortage" in general.
What's really going on is a tank shortage in party/duty finder specifically.

It's easy to find tanks to do stuff with your linkshell or free company.
But it's an entirely different story in party/duty finder.

Why?
IMO, it's because as a tank you're putting your life in the hands of the healer (you can do surprisingly little to ensure your own survival) and it becomes much more tedious to hold aggro on many mobs at once if the dps don't communicate and focus target.
So if I have a choice, I'll choose to tank for linkshell/free company parties EVERY SINGLE TIME over party/duty finder.

Square had the right idea in introducing additional rewards for tanks to entice them into duty finder groups, but the rewards are frankly insulting. All they need to do is increase the extra gil earned to a respectable amount and the tank problem in Duty finder will go away.

As far as party finder groups are concerned, (and specifically party finder groups for extreme primals...) I think the issue is that none of the extreme primal accessory drops are actually BiS for tanks. So over the course of the first couple weeks, tanks got their weekly win in order to gear up secondary jobs with EX Primal weapons and now they simply have no reason to go as tank. SE should have spent more time thinking about the secondary stats on the extreme primal accessories to make them desirable for tanks. And since myth is so easy to cap now with daily roulettes, the mythology reward isn't enough to keep tanks interested in these fights.