Prod tends to have less privileges than CI/CD. CI/CD tends to be full admin, so it's far more sensitive. Prod tends to have tooling for detecting breaches, better logging, etc. People tend to use containers, which act as a sandbox.

Prod also won't be wormable the way that CI/CD is. With CI/CD I can own another dev, use their creds to push another malicious build script, etc. "Attacker is in my prod env" isn't wormable.

Yes, capabilities in prod would be hugely beneficial but removing CI/CD is massive as a win.