Who made all these different levels of Admiral and General?

It all started with a big caveman who ran into a bigger caveman with a bigger club

Good question, the one that always got me was Rear Admiral, get rid of lower half and bring back Commodore. Sounds better I think, but I'm not a Sailor.

Brigadier General makes sense, other military's just call it Brigadier, they typically used to lead brigades. It was the same for us when we still utilized regiments regularly but things change with time.