A lot of stuff is built on the "worse is better" philosophy. Introducing a daemon like yours will complicate the OS a little bit more. So instead, OS developers

A lot of stuff is built on the "worse is better" philosophy. Introducing a daemon like yours will complicate the OS a little bit more. So instead, OS developers outsource the complexity to the apps, and only implement a quick hack to make this possible. In practice, the running-as-root-then-drop-privilege pattern works well enough so nobody bothered making a more complex, but potentially more secure, solution.