If anything, this is a good reason for a DM to curate what's available in the campaign! (cue the inevitable criticism from entitled players demanding to play what they want to play)
A bit more seriously, playing
Keep on the Borderlands always kind of drives home the idea that there are and have been too many variations on the same thing in D&D... and they proliferated for years. Exactly why we have goblins, kobolds, orcs, hobgoblins, norkers, bugbears, and gnolls, why we have so many different flavors of elves and dwarves, ghasts and ghouls, and about a zillion other variations?
I at least partly blame the idea that variations need mechanical differentiation to be meaningful to game designers (and certain kinds of players). I also blame the drive to publish new stuff all the time that previous editions used to deal with - how many monster books can we pad full of creatures to keep the lights on at the company?