The mean can be calculated from the total: just divide by the population. That’s what the commenter to whom you replied did. The total was probably easy to estimate, as the largest healthcare providers’ revenues are in their public filings.
Calculating the median requires individual data. That is much harder to get because, in many cases, no one except the patient has the data.
Because medians don't let you extrapolate total costs or how population-wide changes will affect total costs. The tails of the distribution matter when we're talking about public health.
