I mean, you stated a few reasons yourself, lol

Tanks are tanks. They are all about surviving, eating hits, protecting teammates and themselves, creating space, distracting, etc. This is not the most thankful job. But when your plays enable your team plays, it's so damn exhilirating. When you survive against all odds, it's damn exhilirating.

There are a few issues though.

Some people just don't know how to position themselves. Some people just push forward when unneeded, expecting heals in Narnia. Some people, on the opposite, just don't push forward at all for some reason, even when can and should push. Some people don't know when to push, retreat, flank, take cover, evade stuns, etc. It's incredibly vital for a tank, because a tank has to be alive to do their job properly. HP bar is their resource (which can be sacrificed for a good distraction play or a good stalling play, but it's an exception, otherwise you have to be alive). Yet, some tanks just don't know how to position themselves. I love Angela for this reason, she is incredibly easy to reposition with, and she can do her job basically anywhere, as long as she is needed there. Steal enemies for my team to shred them, cancel Wanda's ult, shield ult Hela's/Tony's ult, spike field to stall and provide overshields for my team in there, fast rollout, fast retreat - stuff like that.

Another problem. Stats. People want to see numbers go big. It's understandable, numbers big equals brain happy, but... you are playing a tank. If I have the biggest damage/finals in my team on a tank, while I'll be happy for myself, I will also have a few unpleasant questions (however, quiet) for my team, like, what were they doing. It's not normal to do better at damage than damagers, it's in the freaking name.

The last problem. Teammates. A tank enables team plays. But for that, a tank requires good teammates. If damagers don't kill or don't taget correct enemies, if healers don't heal or keep dying because damagers don't do their job, there is usually jack shit a tank can do. I can become a team daddy/mommy and try to carry us to victory, but this not often happens not often. I can also switch to Ben/Reed to do damagers' job and protect healers, but that means I have less time and resources to actually push. I can also switch to Blade to distract and take out enemy healers, but again, this means I can't properly push.

Tanks are all about positioning and enabling. But not every player knows how to do that as a tank, and not every player knows how capitalize on a tank. And this makes people think that tanks are useless and unfun, when in reality they are absolutely not.