good try man, but to get a full understanding you need to get deep into organizational theory/system theory or have already the background which I doubt most have here. Again, it would be a longer post, which would lead to a debate because of some unpopular opinions involved and I just don't have the time. Educating you doesn't pay my rent. But you are smart, check out the hints I just gave and write down your guess and be brief (as you requested yourself). People would appreciate this.
meta: people who want to know salaries of their peers and demand equality have a bit a wrong view on companies.
Companies are anything but not about equality, playing fair or being nice to employees. Paying your staff different salaries is crucial and key to run a stable organization. Most won't get why different salaries on the same level create a stable organization, it's complex and would need an extensive/scientific article.
An organization's owner's goal is not to make the individual one happy but rather keeping the entire org on a good track. This involves keeping salaries inequal. Maybe some of you have an educated guess on why inequality creates downright stability.
Possible answer: Because you never ran a company and might lack empathy?
This subject is more complex and just questioning something from a personal, individual perspective won't give you an answer.