Uh, who cares? We know that there are alien species that have incredibly long lifespans (Chewbacca?), so it’s not “unrealistic” by the parameters of this fictional universe that he could still be flying. Also, we have no idea how much confrontation there has been by way of dogfighting between the various iterations of Empire and Rebellion since the death of the Emperor, so maybe he hasn’t been in that many dogfights since Endor. And maybe he’s a really good pilot.

And I repeat, who cares?