Critters exist, whether in the real world or a fantasy world, to fit certain ecological niches - for both flora and fauna.
The various D&D critters, via Monster Manuals or whatever they call them now in 5.0, fulfilled this admirably.
As noted, when Larian determines what type of "niche" critter is needed for a certain area, they need only consult available material to find the appropriate critter (Boss mobs and other specials are always exceptions.) When they veer widely from this material, it creates unnecessary lore conflicts. It's as if they put "Shark-People" into a desert clime - instead of using "Snake-people" , or placed camels in a tropical environment- it defies common sense. Lions hunt in packs, while spiders walk across the ceiling, not the other way around.
And while we are at it, my sword does NOT fire arrows and my longbow is NOT used to bash gargoyles over the head - just in case you were wondering Larian....
One of my fondest memories of the old Gold Box games, involved my party entering the cavernous lair of low level critters - kobolds as I recall - where it was immediately riddled with several rounds of arrows. The kobolds had no need of massive ac or hp - those arrows created the sense of immediate danger - and subsequently whittling down the kobold masses bit by bit, added to the suspense. They were still wimp ass, 3 hp kobolds, with an ac of probably 6 or 7 (back in the old THACO days) but the large numbers armed with ranged weapons kept my level 3 or 4 party (with higher hp and ac) busy and the fight satisfying. Exploding, flammable, poisoned arrows were unnecessary.
