The reason why small and large companies use github (or any other SaaS) is simple: building and maintaining that service is not something that will help that company move forward. Spending the amount of time to build it, integrate everything, maintain all those integrations is time they could have spent building whatever software their company actually sells.

With the advent of AI, people are probably thinking its easy to clone services like this and many others. Sure a 1.0 might be easy to stand up but you've got to now patch it, maintain it, extend it, integrate more tools into it, and keep it running better than github. People may joke/sneer at uptime metrics but when internal tools at a software company are down or worse, buggy and causing issues with delivery, its more painful than checking a status page to see when its back.