> Why would you think that just because nobody negotiated, somehow everyone doing the same job will be making the same amount?
well, at least from personal experience, I'm talking about the kind of company that explicitly says "we want to make you an offer as a software engineer, at such and such level, and this is the fixed salary for that job+level". they could have been lying of course, but once you start going down that rabbit hole you might as well not take the offer.
You are confusing lack of negotiation with transparent compensation, which is extremely rare in the US (outside the government).