That's not really true. As a strictly internal engine they gain the massive benefit that the only users are themselves. They don't have to make affordances or adjust for third party users. If they released it with no intention to play nice with the community then all they open themselves up to is criticism and bad press when they inevitably break third party users. To keep their nice internal workflows without more engineering effort would make the release a dump of the source tree and a "have fun" message pledging zero support.
Often in internal engines there are lots of workflows and tools that only work with access to the company's internal network. Things you can't make public. I work on (a different) internal AAA engine and our build system would not work without access to our internal network even if you had a dump of the entire source code. Do you rip that out or modify your internal workflows? Not so free to release it anymore.