[miksuv]

That's just partly true. From the Base Web website:

"Base Web is a set of reusable React components that implements the Base Web design language and can be used in two different ways:

To build an application that fully adopts the Base Web design language, you import and use Base Web components out of the box.

To build a new design system inherited from the Base, you take Base Web components and customize them through the Overrides mechanism.

If you are building an application using the Base Web design language (the first scenario), you should avoid further customization. This helps to keep the design of your application consistent and makes future upgrades easier."

Uber uses Base Web in both ways since there are some additional related design systems (historic reasons) and that's why a lot of effort was put into overrides.