You're talking as if securing a backdoor with public cryptography is some unimaginable feat of technology.
It's literally a couple hours work.
It's literally a couple hours work.
So, while writing the exploit might be a couple of hours work, actually pulling it off is quite a bit more difficult.
For many projects, that is enough to become the main contributor.
Theoretically, I could even imagine that public key security motivated by some "mostly benign" actor who fell in love with the question "could I pull this off?" dreading the scenario of their future hacker superpower falling in the wrong hands. It's not entirely unthinkable that an alternative timeline where this was never discovered would only ever see this superbackdoor getting used for harmless pranks, winning bets and the like. In this unlikely scenario, the discovery, through the state actor and crime syndicate activities undoubtedly inspired by the discovery, would paradoxically lead to a less safe computing world than the alternative timeline where the person with the key had free RCE almost everywhere.
That's to say an enterprise router or switch would likely have secured exploits put there by corporate and national security agencies, whereas open source exploits would benefit from the probable deniability.
"It seemed like a good idea at the time" is a pretty common story though. ;)