Because grabbing a random prefix from the pool is easier than remembering which prefix is assigned to which subscriber account and keeping it static through ISP network changes.
My ISP does ‘sticky’ prefixes, which means they change when they move users between BNGs but otherwise don’t.
They’ve done new lighting and stuff like DLSS that applies to the entire game, and they’ve also done a bunch of new textures and art assets for some of the game.