I bet in the majority of cases, there's no need to pressure for merging.

In a big company it's much easier to slip it in. Code seemingly less relevant for security is often not reviewed by a lot of people. Also, often people don't really care and just sign it off without a closer look.

And when it's merged, no one will ever look at it again, other than with FOSS.

I think you nailed it.

An insider could just be tasked to look for exploitable vulnerabilities in existing code and compile this information for outside entities without ever having to risk inserting a purpose-made backdoor. Considering the security state of most large codebases, there would be a bottomless well of them.

Who wants this job, that is capable of actually doing it properly?

I've read about workplaces that were compromised with multiple people - they would hire a compromised manager, who would then install one or two developers, and shape the environment for them to prevent discovery, which would make these kind of exploits trivial.

so, Office Space?