Tell that to someone using G Suites! I don't know the real internal structure, but I've experienced a frustrating number of "unsupported features" or "unexpected errors."
Those UX complaints wouldn't exist if Google Accounts were as internally unified as you make them out to be. With a unified set of internal tools and APIs for teams to work with and make sure their product works.
Meanwhile, it took YouTube more than ten years to be able to support not just the default user. BigQuery is 9 years this year, and can't support multiple users in the browser etc. etc.
It gets worse than that.
Because they are still kinda unified. You get blocked for whatever reason on YouTube? Oh. Say goodbye to the rest of Google properties: play store, your dev account etc.
Behind the scenes, a Google account is a single unified Google account, as far as I know.
(In other words, individual products can be confusing, but I don't think the backend inherently prevents unified billing.)