Creating a UI component library is a common practice nowadays. There are many good examples out there, being bootstrap and foundation some of the most popular as of this writing. Although those are good options for prototyping, there are some reasons while they might not work for big companies.
They need customization to match your company image; If you want to use them for your company you might end up tweaking it so much that it no longer look like the original library.
They are bloated; They are made to meet multiple needs so they include many components that you might not need. Even when you can create your custom build, the components themselves are made for flexibility so they will most likely include stuff you don’t need too.