Uber is famous for NIH syndrome. You can tell by their open source projects they've basically built every part of their infra from scratch. So it's not just the application code but everything else that helps run it.
I personally find uberFX to be a fantastic project. It isn't necessary for you to write golang with it, but it certainly does provide a great framework for organizing code so that you can ensure that writing tests is as easy as it can be.
Basically exactly this. Also, a lot of their "in production" open source projects are not "in production" but were generated and released as part of their broken promotion process.