This is the biggest reason why Tanks and Healers are designed to be braindead easy.

[Renathras]

I have a serious question:

Generally? Role and rate of push for content/comfort zone of players. Or to put it another way, the difference between making someone uncomfortable and merely inconveniencing someone GENERALLY.

The Tank's ultimately responsible for controlling all of the enemies on the field/drawing their agro, as well as using their abilities to survive the hits and prevent themselves from dying and the party from taking damage/wiping.

Meaning when they've pulled all they feel comfortable with and you pull more onto them, you're pushing them outside of their comfort zone. In life, it's generally considered rude to push people you don't know out of their comfort zone. It's like walking into someone's personal space, clearly making them uncomfortable, and standing there even when it's obvious you're doing so. Not to be too blunt, but it's something that people without...social difficulties?...learn not to do to other people by the time they get to be adults.

On the other hand, not using AOEs/doing wall-to-wall pulls/etc is GENERALLY (will get back to this) not making the DPS players uncomfortable/pushing them out of their comfort zone, it's just going slower, making the impatient feel inconvenienced. Now, I say GENERALLY, because most MMOs' single target rotations can be performed (or mostly performed) on DPSers. FFXIV has moved towards a lot of single-target things doing AOE damage now. This is a relatively new thing, though. For instance, in ShB, RDM's burst combo was single target only. Holy/Flare/Scorch became AOE in EW. SMN's Bahamut has always been AOE when it comes out, but it used to have a single target version in SB/ShB (Phoenix used both, however). But we're talking a general perception here.

There's also a part that the Tank may be considering the Healer (or an over-concern for the Healer), etc. Tanks watching Healer's MP bars before pulling more things has long been an MMO staple, again, until relatively recently in MMO history.

I'm not sure the best way to describe it other than "social graces"; it's the difference between making someone uncomfortable/making more work for something against their will and merely inconveniencing someone by going a little slower. Like the difference between standing up in someone's face that's clearly uncomfortable with it vs walking down a sidewalk slowly with the person behind you being annoyed you aren't going faster. One is rude, the other merely a natural inconvenience of dealing with other people in a social setting.

Again, as a general thing.

The proliferation of "single target is now AOE" changes the mechanics somewhat, but keep in mind this mindset is 20 years old at this point, and so predates that modern innovation.

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I know it's not an answer that people may like, but to a serious question, that's the serious answer.

EDIT:

In super short, it comes from an older time in MMOs where mechanics were a bit different and people were generally more polite/social and less in a hurry/caught up with the hustle and bustle of modern life to the expense of social graces, so to speak.