Working with real countries is a chore when the game needs to be sold globally. The game loses identity when it cannot identify any of the concrete reasons for the conflict or let any opponents be demonized.
Gameplay will need to justify crazy shit to let people do fun things like fly around, revive teammates from the dead, or drive giga-vehicles. Fictional worlds are perfect for that; and they can still choose to have damage models influenced by military tactics games.
Fantasy worlds can establish a visual uniqueness that conveys appeal for the game in its marketing, without the brand getting confused with others involving “tough soldier holding assault carbine while backed up by a tank”.
EDIT: One more. Fictional firearms don’t give licensing money to the manufacturers of real arms.

My other, separate hope is for a Battlefield game that rewards squad play even when the players in question are not amazing crackshots. They’ve aimed for that many times, but generally I only see campers and lone Rambo wolves win these games.