As a simulation, making an ally’s space count as difficult terrain makes sense, but this rule did not exist in earlier editions. The fifth-edition designers aimed to make the game streamlined, so why introduce this new complication? The designers made allies count as difficult terrain to deal with the side effects of allowing characters to move, attack, and then use their remaining movement. The rule limits the implausible situation where characters move to the front of a line of allies to make an attack, and then move to the back of the line, all in the same turn.

We all understand that turns serve as an abstraction to make the game playable, and in the game world, most of the action in a round all occurs simultaneously. Allowing each member of the party to squeeze up, make an attack, then retreat to the back, leaving the tank in front to face the enemy’s attack, exposes a certain absurdity in the abstraction.

Apparently, an enemy’s single opportunity attack fails to do enough to discourage the batter-up attack queue.