It uses components, it is not built on a component - the goal is for it to be able to manage multiple modals and some situations that are impossible to do right with component-based modals such as reusing a backdrop (I argue that multiple modals is a bad pattern, but it is a high demand user feature). It also allows flexibility into hooking into the lifecycle in the future with knowing when the modal renders, starts or finishes animation, and similarly for when the modal closes.

With component-based modals, you end up with bad APIs for these use cases, as well as less control over your state.