Why?
D&D what good comes from playing monsters in such a manner? If a monster is mart enough to kill a player, why wouldn't they? What benefit comes to a monster from spreading out his damage?
The game is more fun when there is an actual sense of danger in each combat. And monsters should be played to their intelligence. Any creature with any modicum of intelligence should know that removing a creature from combat is much better than spreading out damage. And any creature with even a little bit of intelligence should know that killing a creature to prevent it from being healed mid combat is probably a better choice than leaving it breathing and able to fight again.
Sure, playing with such tactics makes combat more deadly, but is that really much of a problem. In general 5e combat is rather easy anyway.
Playing the monsters like monsters?
A big, powerful monster like, say, an earth elemental won't distinguish between a armored human with a sword and a human wearing just clothes. It won't notice a few extra sword swipes. If it's coming at the players with hostile intent, its coming at the players
as a group. It doesn't want to wear them down one by one like a tactically-minded group of humanoids used to fighting threats that are bigger or more numerous (sometimes both at once!) than themselves. It's a
big thing and it wants these little things
gone. If the earth elemental wants to just drive them off, maybe hitting them all once will make them go away. It's easier for the puny humans go away if they are all wounded, but all walking. Feel free to have to have the monster switch up tactics and start focusing down a character after they have
earned the monster's attention. Baddies get enough hit points compared to the party in 5th that they can afford to play less-than-perfectly in the opening stages of the fight.
On the mechanical side, I think damage is more meaningful when it gets spread out. I'm not playing to
win, I'm playing to
challenge. You might consider those to be the same thing, but I disagree.