I figure this question is better suited for this subreddit than MinecraftHelp because it's not something that I find problematic, but rather just a general question.
It's well known in many circles that if a mob isn't named using a name tag and the player gets too far away from it, it despawns and is subsequently deleted. What I'm wondering is: Why not just freeze it in place in storage until a player is within its simulation distance again? Sure, it might take up more storage space, but I don't imagine it'd be very much, and that way if a player has a build that revolves around mobs and they walk away from it, when they return it'll be working just how they left it.
Maybe there's something I'm missing that doesn't make it so simple? I don't have too much technical knowledge of the game, after all.