The onus is NOT only on the DM. I mean, if the DM is the controlling "this is my world" type, then yes, I guess it would be. But that isn't the only way to play the game. As several have stated, over and over again, in this thread and other similar threads.
A DM who collaboratively builds the campaign world with their players can most certainly lean on those players to help decide . . . where do turtle people come from in this world? What is their culture like? And for tortles at least, you don't need to design any player options, as it already exists.
Of course, if I wanted to play a lupin (dog-people from BECMI D&D), we'd have to cobble together something. Or modify the dragonborn to represent draconians in a Dragonlance campaign. But that is also something the DM can collaborate on with interested players. It's actually quite easy, less work, and a LOT more fun . . . well, for me at least.
It seems folks arguing against this playstyle have little trust in their players to collaborate responsibly and are loathe to give up control over THEIR world . . . and again, that's a traditional way to approach the game, works for a lot of folks, but for an increasing number of us . . . no longer is attractive.