There's two reasons.

1. Gerrit's approach requires a stable Change-Id in the commits. So it doesn't just work out of box with stock git. It requires that the submitter's git configuration and the repository be set up to support this. (Note that JJ supports this out of the box)

2. Cargo cult. We have a whole generation of software developers who grew up in this generation, love GitHub, and have never known anything else. The "PR" approach is considered orthodox. Unless they went and worked at a Google or somewhere like that, they've probably never been exposed to alternative processes.