Or you can play the public hero and end up best buddies with Snowden and exiled from every Western country. The governments don't particularly like whistle blowers.
And if you found a backdoor, you probably wouldn't want to use it on every device all at once and reveal its existence. Somebody somewhere will log it. But if you carefully pick and choose your targets, just a few in a million (or several billion), it might not be detected for a long time.
White hack groups like Project Zero routinely find these exploits. That doesn't necessarily mean they're planted, it just means that it's definitely possible to hide them from common view, and it takes a lot of skill and dedication to uncover that, then also a strong enough corporate shield to protect you from any possible fallout.
Unlike taking pictures of UFOs, it's not just being in the right place at the right time. It takes a very high level of skill that the general population doesn't have.
It's like what intelligent skeptics keep saying in the ongoing discussion of UFOs / UAPs and claims of extraterrestrial visits: With everyone having a camera in their pocket (or, more likely, in their hand) these days, shouldn't there be more compelling photographic evidence of extraordinary things at this point, if extraordinary things are really happening?
Edited to add: I guess I said it pretty well further down in that thread, "It does seem like some researcher or journalist should have blown the case open by now if this thing were systematically providing telemetry from everyone's "powered off" (but still plugged in) machines to an intelligence agency."