While that's true, there's no shortage of players playing tanking classes or tank equipment dropping in the random dungeons and raids they don't tank for. Those players could just as easily build tank sets and queue for tank spots if they have a spec for it. There's still plenty of opportunities for them to build and use modest tanking sets. So, it's not for lack of opportunity. I think there are 2 specific problems:
1) Those who may be interested in tanking don't have another spec to create. For pallies and druids, healing specs are very viable. Healers for raids have been, are, and always will be in short supply, whereas, as you said, there's a glut of tanks in raid content. Pretty much everyone has a DPS spec, and if they also have a PvP or healing spec, there's no room for a tank spec.
2) Tanking has a high learning curve (unless you're a pally) and is the most punishing for failure. Especially in BC onward, if the tank makes a mistake, it probably means someone will die (if people are wearing gear at the instance's level). That is intimidating.