Right, I'm sure the BP rebrand included new expensive signs at all of their gas stations. I isn't like the logo design walked off with hundreds of millions.
The designer does the rebranding, not the customer. This means generating all the new style guides, promotional material templates, business cards, even sometimes ad templates.
All of that derives from the logo/wordmark, but it's usually the majority of the work.
I've read that the actual logo itself was only something like $4.6m. Which still seems like a ton really, but I guess easily worth it for a company of that scale.
Creating the logo is only one very small piece of a rebrand.