What if I plant a hook on the engineer’s laptop that will slightly modify their commits in such a way that eventually I smuggle malicious code over? First, exfiltrate the code over to me, learn from previous commits, start poisoning. Stamped by the CI, no less. In the days of slop PRs thousands of lines each shouldn’t be even too hard.

The point is that at some point you do need to open the door and let the code in.