[IshKebab]

This happens all the time because setting up an entirely new domain yourself is way less work than asking the internal IT team to set up a subdomain for you. If the GitHub IT team is reading this then yes, that means you failed.

[tomschlick]

I'd imagine it's less about setup complexity and more about reducing the attack surface of the main domain where any number of mistakes on the subdomain could expose a vulnerability for the main domain as well.

[IshKebab]

Not in my experience. It's about avoiding bureaucracy.