Snarf Zagyg
Notorious Liquefactionist
The issue isn't explaining exotic races in your setting.
It's getting your players to like the explanation.
"All the uncommon and rare races are refuges" might work for some people. Other people might see this as lame or and excuse to roleplay an exotic race like a walking caricature.
Because if the DM doesn't really care how the they make the exotic races fit into their world, they can't really complain about how the player roleplays them.
So, a few issues that I see, IMO.
The first is one that might be peculiar to the area I am in, but seems to be somewhat common. Despite the fact that enworld attracts DMs like Citizen Kane attracts proponents of the auteur theory, there tends to be a lot more people that want to play D&D than want to run D&D. Which means that, for the most part, the issue really is explaining the exotic races.
In other words, it's about the work involved. If a DM is running the game, and creating a setting, they may not want the additional cognitive work, or any additional work, in adding more exotic races and explanations into the game.
In many (most?) cases, this might not be a big deal. In a kitchen-sink setting, it can just be ignored. After all, who would notice one more alien in the background of Mos Eisley? On the other hand, if the setting is more themed, or has more tight integration, politics, etc., it might be more difficult.
I think the formulation in your last sentence is interesting. Couldn't it also be stated, "If the players can't be bothered to roleplay exotic races well, then the DM shouldn't be bothered to make them fit into their world?" Is either that formulation or yours accurate?
I reminded of what I wrote in my conclusion-
For the most part, I think the fault lines tend to go down the usual sides- it's the whole "DM sets the rules and strictures for the campaign" v. "Player Agency, you don't tell ME what to do" arguments that get repeated, just with a new coat.