I think the reason why I can only play myself (or one with my exact personality) in any given interactive media is because I lack virtualization ability.
I can empathize with another person-- by literally running what I predict they experience through my mind. It's "virtualization" by way of chroot/Docker, basically. The person I'm trying to empathize with needs to be similar enough to me, otherwise I stop being able to construct a suitable environment to simulate them. I try to empathize with as large a range of people as possible, but even then I have limits. No dedicated virtualization capabilities needed. I trust that whatever's in there isn't malicious, which is probably a safe assumption to make, considering I made it.
But I cannot, say, maintain the idea that I play a different person from who I am. Not possible. I can't simulate entire separate personalities because I'd run out of resources. Memory, CPU, I/O, it's probably all 3. It's dead slow. I simply can't do it on any practical scale. You ask me to play as this evil motherfucker who supports Cesar's Legion in Fallout: New Vegas, I'd find myself supporting literally anyone else. I am who I am: me, and all of my characters are self-inserts to the fullest possible extent.
On one hand, it makes me genuine (since I literally can't express a personality that isn't this one, only suppress parts of it), but it also means that I can't do creative writing. I'd keep making multiple self-inserts by accident, so it'd just be a "Me, myself and I" situation, just many mes interacting with each other. I'll stick to writing technical manuals of my discoveries instead.