Not your screen monkey (he/him)
I don't think it's literally the pool of hit points, per se. Rather, it's the high degree of defenses before the unit targeted, whether PC or monster, can no longer take actions on the battlefield. Those defenses could be a lot of hit points, a high armor class, high saves, lots of STUN with high PD/ED, etc. Pretty much any defense that requires multiple attacks to either ablate those defenses or pierce those defenses with multiple attempts, barring a lucky initial shot.Focus fire isn't the problem, focus fire is the solution to the problem that monsters and PCs in D&D have huge pools of Hit Points that need to be whittled down to 0 before any of the attacks that hit actually matter.
And, honestly, that's pretty hard to design away, for most RPGs. And that's assuming we even want to do so since a whole lot of people also seem to want to avoid taking out BBEGs with a single shot, so they need some kind of ablative defenses.
I think there is some potential here. And I think making sure that all units on the board pose some degree of threat helps as well. Bounded accuracy has been helping me with that because the PC ACs in the groups I'm running for are all in reach for most of their opponents. They don't post strong threats, but they do wear the PCs down and are worth taking out. They also don't necessarily go down in one hit, meaning they can keep the attention of the players longer.If you want to encourage spreading attacks around, give them more effects than plain damage, so that attacking multiple targets can actually be useful in controlling the battlefield.