At Google specifically, even being promoted to staff is a huge undertaking. And until recently, there was an expectation of forward career trajectory built into the lower ranks, i.e. every engineer was functionally multi-year probationary. If you found something valuable to do but you weren't progressing your career (because, say, the work was necessary but boring, like micro-optimizations, feature polish on a mature product, or documentation / example creation), you'd start to have talks with your manager about your future at the company.

I believe they relaxed that process when someone at the top took a look at their org-chart and realized they've become a big company where they need a critical mass of not-actually-interested-in-progressing engineers to keep the lights on and if they actually followed their policy, they risked churning those reliable workhorses out of the company because they couldn't actually afford to find a slot to promote them all.