Academia used to be much more politically balanced: https://www.independent.org/tir/2022-23-winter/the-hyperpoli...
It would be very illuminating to compare academics' political leanings in other countries as well, but I am not aware of any such studies.
Now it is basically officially banned to hire conservatives: Diversity Statements Required for One-Fifth of Academic Jobs - https://www.schoolinfosystem.org/2021/11/11/study-diversity-...
Even if hired, bias persists: The authors also submitted different test studies to different peer-review boards. The methodology was identical, and the variable was that the purported findings either went for, or against, the liberal worldview (for example, one found evidence of discrimination against minority groups, and another found evidence of "reverse discrimination" against straight white males). Despite equal methodological strengths, the studies that went against the liberal worldview were criticized and rejected, and those that went with it were not. - https://theweek.com/articles/441474/how-academias-liberal-bi...
When polled, 4 in 10 academics admit to discriminating against conservatives: https://www.cspicenter.com/p/academic-freedom-in-crisis-puni...
> So why are we [..] assuming there’s a problem with equality of opportunity?
To answer your question: we're not assuming anything - they have official policies to that effect, they openly say so when polled, and it has been measured in studies.