If you trace its history, the commit for it was made in June 2014. Since that point in time, it's stayed disabled by requests from multiple scripters. So technically it only took 2 years overcome that consensus and finally enable something that's been around, simply disabled.
We have an fairly relaxed culture and we wanted to keep it around, hence the hesitancy around it.
If you think that it'll get rid of manual code reviews, you need to re-assess how you think security. It won't replace manual code reviews.
Your thinking is way too closed minded. The key to security is looking at every angle. You're just not thinking about everything and I'm certainly not going to hand ideas out for people to use. A SecurityManager alone won't be getting rid of manual reviews, that's for sure.
Edit:
Since it wasn't clear, please note that I mentioned "security systems" in the original post, not specifically a SecurityManager. So for clarity purposes, yes, manual code reviews could be decreased as part of the broader security system, but it is unlikely that they will be completely gone. A SecurityManager alone won't cut it.