It's the way Bethesda designs their games, as building blocks that snap together, which means reusing textures and meshes a hell of alot. It's been the same way since Arena really. Though instead of being randomly put together from an algorithm, they are instead put together by the level designers. I'm still impressed with how they can compress all that audio though. I just hope the atmospheric music isn't compressed that much, though the radio audio can be compressed all they want because it's suppose to sound like that.
I also think a big motivation for them compressing Fallout 4 as much as they can is leaving room for console mods on console users' hard drives. Which is a real concern now days given how big games are this gen, especially with Sony not allowing use of external storage devices that aren't only used for backups.