Just HP stacking on enemies doesn't equate to an increase in difficulty

[CazzT]

Technically, it does.

More time spent killing stuff = more chances to fail mechanics.

Hard to mess up mechanics if you don't see them 'cause you zerged through the fight in 0.1s.

That and it gives a sense of difficulty without actually making things mechanically difficult, so that horribad players don't get butthurt when they fail constantly. They just think "Look at how good I am, I defeated this difficult boss that took like 3 hours to beat!"

That's assuming the mechanics are are easy to fail. Outside of raid content, you practically have to be AFK for the mechanics to be of any legitimate concern. Stacking HP just means having to barely pay attention for another minute. That's not difficulty.

Making a bullet sponge boss doesn't make the enemy difficult. It just makes them tedious. Tedium is not difficulty. (unless you count "fighting boredom" to be part of the difficulty calculation)