The people who made Gmail are either still working on Gmail, working somewhere else, or working on a pet project because they bought the proof-of-competence to choose their project. Google's management structure basically doesn't have anything that says "Hey, you were successful at X, can you work on (thing adjacent to X)?" and incentivize the employee to do that if the employee wants to do something else.

There's no reason to assume the people working on Slides, Spreadsheets, Drive, Docs, &c started particularly overlapped (though I'm sure there's consolidation these days). Similarly with GCloud; all the pieces of GCloud started as independent initiatives (App Engine, Cloud Storage, BigQuery, Compute Engine, &c). All of these started separate and only began using consolidated resources / providing consolidated UX frontends and APIs as they were forced to by a management chain ad-hoc'd together after Google decided "Cloud" was a space they wanted to do business in as an organized front.