Firstly, why just Google engineers? Many Google employees and any 3rd parties Google share data with would be included.

Secondly, that's a single step in the threat model; it's not just about what a Google engineer would do with your data, it's about attack surface area when any Googlers (or Google infra, or Google partner infra) that is compromised automatically exposes you.

The simple act of transmitting and storing your data to anyone, no matter how secure their systems are, is still by definition less secure than simply not transmitting that data.