That's definitely a problem with every bit of code, that everyone relies on stuff they don't or can't review.

My point is that FOSS provides a false sense of security ("Millions of people use this library. Someone will already have reviewed it.").

But the bigger issue is that FOSS is massively underfunded. If OpenSSL was for-profit, it would be a corporate project with dozens if not hundreds of developers. Nobody would buy a piece of core security infrastructure from a self-employed dude working away in his basement. That would be ridiculous to even think about that. And if this standard component was for-profit, even very low license fees would generate huge amounts of revenue (because it's used in so many places) and this would allow for more developers to be employed.

And since it would be an actual thing that companies would actually buy, they'd demand that third-party security audits of the software would be done, like on any paid-for software that companies use. They'd also demand some SLA contracts that would hold this fictional for-profit OpenSSL accountable for vulnerabilities.

But since it's FOSS, nobody cares. Companies just use it, nobody donates. It's for free, so the decision to use it usually doesn't even go through procurement and anything related to it. I tried to get my old company to donate to OpenSSL in the wake of Heartbleed, and the company said they don't have a process to donate to something, so can't be done.

So everyone just uses this little project created by one solitary hero and nobody pays for it. And so that dude works alone in his basement, with literally the online security of the whole world resting on his shoulders.

Luckily after Heartbleed a lot of large corporations started to donate to OpenSSL, but there are hundreds of other equally important projects that still nobody cares about. As seen e.g. with the .xz near miss.