Pug tanks and healers are a thing too. Not everyone is fortunate enough to have a group of folks to run around with. Some people are casuals on top of that.
You can climb pretty easily to Mythic+ 6 being whatever level of player. After that players get pretty specific with their wants. Tanks and healers have a bit more leniency. DPS not so much. Players want either someone of recognition or whatever’s going to clear that run faster.
Granted players honestly aren’t that stingy. The issue is the wait time. You can get passed up by group after group because someone more powerful/FOTM came around.
At least with a tank spec you could get accepted into a group, boost your ilvl, and try again as DPS for that Mythic level you’ve been trying to get.
How can it be a myth if there isn’t data to say for or against.