You're thinking of "knowledge" as if it were just things written down. It's much more than that — it's experience, it's judgment, it's habits of mind. Aside from the fact that different teachers teach to differing abilities, different teaches just know different things. To be taught be someone who is truly at the top of their field is just a different experience that someone who is not.

Separately, different universities have different cultures of work. A place like MIT, every student is competing against one another, produces a different kind of student than a more laid back place.

Lastly, one of the main reasons they are treated differently is that universities attract different quality students. The more selective the university, the more valuable the degree. Not because the school necessarily teaches better, but because the degree itself means, "I beat out 1,000 other people for this opportunity and I made it work — I must be great."

There are all sorts of issues with judging the quality of an education on the prestige (or selectivity) of the degree alone, and there are many complications with making judgments about teaching ability (prestigious research universities often grade faculty primarily or only on their research, and so they are disincentivized to spend much time working on their teaching), but the idea that all universities give the same quality education, or produce the same kinds of outcomes, is clearly not the case.