The difference is that Volo's races are noted specifically, still, as monstrous races. They aren't meant to be in the same 'roster' for an average campaign. Any DM can run a game as they wish, but BG3 is representing 5e in its first video game. To misrepresent the lore by allowing goblins as a standard race would be a strange move, imo.
Well seeing as BG3 is useing the Volo`s for the mind flayer transfermations and lore as one of those monstrous races adventurers why not the others I mean page 73 has the tad pool picture they use so volo`s is already a big source for the game why not the 15 playable races people love from the book in the game like i said if you dont like them you dont have to play them or recruit them but many do play as goblins and their clearly is a desire for them or they wouldnt of added them as a playable race Creating characters as creatures normally cast as villains offers up some interesting roleplaying possibilities.
Whether played for comedy, as a tragic story of betrayal
and loss, or as an antihero, a monstrous character gives
a player a chance to take on an unusual challenge in
the campaign some might like the challenge some might like the story so i dont thin its a strange move at all, imo