pytype had two features that made it uniquely suited to google's needs:

1. it had powerful type inference over partially or even completely unannotated code, which meant no one has to go back and annotate the very large pre-type-checking codebase.

2. it had a file-at-a-time architecture which was specifically meant to handle the large monorepo without trying to load an entire dependency tree into memory at once, while still doing cross-module analysis

there were a couple of attempts to get mypy running within google, but the impedance mismatch was just too great.