That acceptance is in my experience due to lack of skills/intelligence. By realising that you don’t have enough skills/intelligence to withstand the (possible) consequences of speaking up, it is rational to comply with the rules and just hope that somebody else will bring the change.
That’s one possibility, but I see others.
in my personal case, my ego was inflated in childhood by words such as “you can do anything” (“if you focus and work hard enough”, but my child brain conveniently ignored the last part). That ego still plays a large role in my dissatisfaction. I think it’s mostly a good thing, though, insofar as it aligns with my other values.
there is genuine, deep joy and happiness to be found in living the life of a ‘loser’ in the terms of the Gervais principle as summed up here: https://www.lesswrong.com/posts/wd7qxFBF2swRscBiS/academia-as-company-hierarchy
That is, people who work in the lower strata of a hierarchy, but whose real life is in the time outside work: friends, family, activities… Maybe there’s a high variability here, so that some people skilled enough to speak up as you say prefer not to because they have amazing friends or an amazing hobby.