Using a separate domain for user generated content is usually done for security reasons. For example, if a user-generated chunk of JavaScript was executed from drive.google.com, then it could potentially gain access to your drive.google.com, or maybe even *.google.com, authentication cookies. Scripts running on an unrelated domain have no such access.

This usually isn't the only thing protecting against this, and is instead used as an additional safeguard.

I believe Google's use of this practice also predates widespread support of Content Security Policy, which isn't to say that this is a useless practice, but perhaps it isn't as important as it used to be.