For some components, like a button, overriding some css may be all you need. But other components can be more "opinionated" - for example, for a Slider component, do you want to label your thumb value? Does it go on the side of the slider or on top? Maybe even embedded into the slider track itself? Is it only visible when you're dragging or always there? What if there are multiple thumbs? Etc.

Some libraries will offer a ton of props to customize different parts of a component - and for some use cases, that is enough. But sometimes it is just easier to structure and style the component however you want, with custom behavior sprinkled in, and then letting a library like react-aria take care of the rest.