Hussar
Legend
True, but, then again, in the first encounter with a 1st level party, smoking one or more PC's in the surprise round isn't much fun for anyone. So, we run up against the old issue of what is "realistic" vs what is "fun". I mean, sure, you shoot the least armored target, get lucky and drop the wizard with one shot. Do you then start shooting the downed PC? It's not like it's likely the goblin can outright kill the PC - although that is possible depending on die rolls.In 1st battle you have 4 goblins in ambush with good stealth(+6) and since they are lying in ambush probably that check should be with advantage.
But, then again, once you get past those first three levels, then that sort of thing becomes less of an issue. At worst, you're just knocking out PC's for a round - then someone stands them back up and in a lot of groups, no one even loses an action.
Let's not forget too that most monsters don't have ranged attacks. Humanoids, sure, but, most of the Monster Manual don't have ranged weapons. Which means that they are going to get up to the front line and then usually blocked in by the tank characters, so, running around the fighters is only an option if you're fighting in open fields. Finding a choke point typically isn't all that hard.
And, finally, lots of monsters aren't smart enough to pick the "squishiest" target. The average ooze doesn't really care very much. Most beasts are likely going to attack the closest thing, rather than running by something to attack in the back.
So, yeah, it really, really does matter what kinds of opponents you generally use.