I suspect it's because it takes time to remove references to Red Hat in packages (licenses, any proprietary bits, etc).
The purpose of RHEL is not to build CentOS. It's to sell RHEL. Red Hat helps with CentOS because CentOS drives business to RHEL. That doesn't mean that Red Hat benefits by diluting their branding. Red Hat didn't become the biggest name in enterprise Linux by not putting their name on the product they're selling.