However, I've watched dozens of online courses taught by CMU/MIT/Stanford/Cornell/etc. There is a huge quality difference between my education and those top-tier schools.
Tenure track basically requires someone to advance to the leading edge of a field (or start their own field) and then become recognized for their research. You have about 5 years to make an impact or you won't get on the tenure track. Being taught by the leading edge researchers is a huge advantage.
So, given a higher quality of teaching excellence, higher standards of work, and access to the leading lights in a field, it seems likely that top-tier school graduates would be much more likely to advance their field. It is not always true. Sometimes tenure becomes "on the job retirement".
Being a professor is a job skill. In any job there is a 'bias' toward top-tier people who have demonstrated success. Look at company CEOs. If they have led a successful company they are much more likely to be chosen to lead over someone new.
There is also the "who you know bias", as in any field. When making a tenure decision would you weigh a recommendation from me or from the professor at a top-tier school who taught you, who you work with, and who you respect?
Sure, there is "bias". But that is true of any top-tier profession. If Dolly Parton said of someone "He's the best singer / songwriter I know", would you hire that person to write your songs?