Why? If someone is willing to offer me X, they value what I can do at X (or more). Why does that need justification if it's some multiple of the median?

Prepare yourself for the downvotes... Every time you suggest that some people are payed too much relative to others in the society which creates massive inequality you have a rush of libertarians trying to justify that it works as expected.

Someone felt that, even at this price, they would be able to profit from my work. Above that, what other justification could there possibly be?

Or, if you are an entrepreneur, your earnings are derived from organizing and selling the work of others. Again, what other justification exists, apart from "this many people wanted to pay me this price, those guys said they would do it for me at a smaller price, and I kept the difference."

Why?

That justification is usually done once, to the employer/client :)