I think you are missing the various mechanics changes in 5e that encourage just the situation
@Ruin Explorer described as making fireball so great in 5e.
5e made in combat movement so easy that combats are no longer a spread out tactical affair. Players all use ranged attacks with no downsides or just close to the most valued target & the GM is expected to direct the monsterts towards the crunchier targets rather than simply becoming an adversarial killer gm that sends the monsters off to geek the mage/squash the cleric. Past editions had things like 5 foot step/shift & movement based AoOs but not 5e, absent any of the mechanics that once supported a more spread out combat the players are truly playing by a different set of rules than the combat as sport monsters. Toss in bounded accuracy & 6-8 encounter expectations to create a situation where the GM is pressured to put a gobton of weaker monsters that cluster around a point of crunchier players that fireball is likely able to easily cover. The players aren't worried about fireballing an ally in such a situation because monsters lack enough tohit & damage to be a real threat & the death save mechanic combined with things like healing word that even an unusual freak case of it actually mattering probably won't really matter.
The only time I've seen friendly fire actually matter in 5e was when a player asked if a cloud effect was flammable & I told him to double the damage when faced with a choice between "Maybe maybe not do you want to use your action to figure out?" vrs "nah I cast fireball since bob's saying
doitdoitdoit" Everyone was shocked when I didn't save level 8ish bob from death by massive damage & there were even efforts to (incorrectly) ruleslawyer it with rules that didn't exist. Players will fireball other players with a shrug & sorry bro we will get you back up. Bob was immediately revivified after the battle concluded.