I've been a tank main for 17 years now, across many mmos. In FFXIV, since 1.0 release. Tank sets the pace because if you have healers or dps going off to pull mobs that you do not know or do not trust, half the time, it can cause a wipe, people get frustrated, point fingers, then it becomes wasted time.
Tank decides what he can handle for pull size, Healer dictates what he can heal for max pull size, DPS dictates how fast they can potentially kill the pull size before tank and healer runs out of cooldowns. So if I do a run with random strangers, have a Sam DPS, RDM DPS, Healer is a scholar who says pull full max. I pull 3 groups in the first pull and barely survived it to where I did perfect cooldown rotation and had to still pop my long CD buff, all because the Scholar didnt maximize his healing, or the DPS was bad since well, SAM aint too good with AoEs and the RDM was instead of AoEing was trying to burn things down one at a time, or a combo of both, then I will start pulling two groups at a time, and if any of them griped I will tell them straight up this group cannot do massive pulls, deal with it.
The problem with a majority of Tanks these days, and why they get yelled and harassed at to a point they give up being tanks instead of persisting and figuring it out, is because tanks are the pointman, they are the leader, they set what can and cannot be handled. Been like this since Antiquity *Everquest* and will be like this till the end of days or the Holy Trinity is completely done away with in mmos. Most playing tanks just are not cut out for it. They get nervous, or paranoid on making mistakes, or simply don't have any confidence. The best tanks are those with a Clear head, Calm, is willing to accept mistakes and do better, willing to be the leader, and most of all, are Humble.
So I will point this out for all the dps and healers out there. Do not rush your tank, do not force him into bad situations, and do not blame him if its something that is your fault, do not chastise him and insult him if he messes up badly, when you should be giving him advice, or very least, ask him if he wishes to know anything if you got the experience to share. Otherwise, the Tank shortage will just get worse and worse, and you will keep getting these healers and dps who get fed up, and tries to then main a tank, and most of them give up after a month or less when they realize it is just not for them.
Their are awesome players out there who has the mentality to play all 3 *some mmos 4, like FFXI* roles and do exceedingly well in them. Their isnt many of these players who can. A role you enjoy can speak volumes on your personality. I know being a tank can seem easy to most, and you are right, it can be a pretty easy job to do. But it requires a certain personality, a certain psyche to do well.
Anyway, ranted enough. But Tanks ultimately set the pace based on what he believes his group can do. And that mentality will always stay.
Its also why I get 2 to 3 commendations, every, single, run.
<- 17+ Year Main tank veteran of countless mmorpg's.