I don't think there is anything preventing them from putting Variant Humans in the game. Larian has not said either way, but as its in the PHB then its likely to be there. It's probably just not a priority to address this right now.
The biggest problem with re-rolls is multiplayer. I don't care what you do in your single player adventures, feel free to cheat yourself of a challenging experience all you want, just keep that shit out of multiplayer. Nobody wants to spend an hour in character creation while four people re-roll for that *perfect* roll. Ugh. Point buy is honestly the system with the most integrity from a gaming perspective.
Finally, humans are definitely not underpowered. Even without variant humans they get the highest stats overall and make the best barbarians currently.
