I don't think people appreciate how much effort Google does to minimizing cookie leakage. It's good security and privacy hygiene, and it also speeds requests -

I don't think people appreciate how much effort Google does to minimizing cookie leakage. It's good security and privacy hygiene, and it also speeds requests - transmitting sizable cookies isn't free, and takes up time, especially when you're paying by the millisecond. This is why there are various different domains for static content, domains for user content (googleusercontent.com), domains for certain cloud services, etc. Protections against XSS and cookie leakage is taken very seriously.