I would explain to you, in fine detail, the many many many many many reasons tanks arent tanking for pugs, how tanking itself is broken, and how there isnt even a true "tank shortage" to begin with.
Alas, I'll leave you with this nugget of wisdom...roll a tank yourself and you'll never need another tank again. I honestly believe that 80% of the people who whine and say tanks are being greedy and selfish and that tanking isnt harder or more stressful than any other job (and come with the most disadvantages in the game) have never ever ever tried it.
If these people would just grind a tank to 70...tanking instances on the way (pugs) and continue that trend into grinding rep til exalted and heroics...tank for pugs at least once a day...go raid and get good gear...and watch as tanking those heroics get harder and harder with every upgrade you get...go try and solo and compare how you do against your DPS toon...go try and PvP and see what an utter waste you are...and how your spot would be better filled with a lvl 65 DPSer in greens than you. Basically, walk a mile in a tank's shoes.
Tanking itself is broken and unrewarding in so many ways...the only real way a tank can improve the situation is the try and remove a few of the removable aspects...and that is pug players. Why take a PoM Pyro mage who thinks pyroblast is a great opener for the mob marked as X and that sheep must have something to do with the hunter's pet...the hunter who, incidently, standing at the tank's side whacking the mob with his weapon "because he forgot arrows"...meanwhile, your healer is still sitting on the ground...afk without telling anyone. Of course, you all die and as you are running back, the rogue asked what happened...it seems he was down the hall pickpocketing mobs and totally didnt even see you all were fighting.
Of course, that isnt to say tanks dont have shortcomings...but I can almost guarantee you that for every scrub tank who is doing good with just keeping 1 mob on himself...I can find 3 DPSers who would probably could be outplayed by a retarded chimp with 1 hand.
But like I said...if it really bothers you that there are so few tanks for pugs...roll a tank yourself and dedicate your play time to babysitting puggers through ANOTHER non heroic SV run...