Windows Admin Center was created because everyone wanted to manage Windows from a web browser. The current result is it works, but it's far from an optimal GUI solution for managing Windows machines.
Windows Admin Center was created because Microsoft thinks sysadmins want to manage Windows from a web browser. Instead, most of them would rather use desktop apps over RDP, which is why the deprecated Microsoft Management Console (MMC) is still widely used. WAC adoption is nowhere near what it should be in practice at this point.