The NSA, the original primary developer of SELinux, released the first version to the open source development community under the GNU GPL on December 22, 2000.[6] The software was merged into the mainline Linux kernel 2.6.0-test3, released on 8 August 2003. Other significant contributors include Red Hat, Network Associates, Secure Computing Corporation, Tresys Technology, and Trusted Computer Solutions.
For people interested in the subject. Read This Is How They Tell Me the World Ends: The Cyberweapons Arms Race
TLDR current day software is based upon codebases that have houndreds of thousands lines of code. Early NSA hacker put forward an idea 100k LoC program will not be free of a hole to exploit.
To be a target of a 0-day you would have to piss off state level actors.