^ this.
Paladins especially are egregious there as it's how the design works out. If you're not low on HP, you're not getting healed. If that's because you're already healing yourself, well, then that is how it is.
Heck, I'd see it as a GOOD sign if at the end of a successful run the tank is top on meters. It means people were actually dodging mechanics and avoiding damage. Many dungeons actually have fairly low overall healing requirements, with phases of relative lull in between some spots of intense healing. That's part of why Druids are so popular in high M+, as they can switch to passable damage dealing at a moment's notice. If your DPS are on point, they will take very little damage a lot of the time, and if the tank then has a lot of self-healing to boot, what's a healer going to do?
Can't heal damage that isn't there.
Now, if you're FAILING on runs because of low healing, that's a wholly different matter. If healers just aren't doing their job, that's on them. Do note also that many healers you find in M+ are actually DPS players that just go healing in order to get into PUGs. They may simply not be very experienced despite their high ilvl. But, again, that's only for the cases where healing is actually an issue and impacts the success of the run.