A junior dev may also swap jobs, require vacation days, perks and can't be scaled up at a the click of a button. There are no such issues with an agent. So, if I were a FANG higher-up, I'd invest quite a bit into training LLM-agents who make pesky humans redundant.
Doing so has low risk, the senior devs may perhaps get fed up and quit, and the company might be a laughing stock on public PRs. But the potential value for is huge.