The phrase comes from hip hop slang where being in the building means arriving and making your presence known, and Kanye West built his public persona around that kind of self assured entrance. Clips of him announcing himself were clipped and shared on meme pages and soundboards, where the short boast became a reusable entrance cue. It stayed popular because the line works without context, fans and editors use it for anyone walking in with main character confidence, from friends to pets to game characters.