[fhars]

Because it is the correct measure for the question at hand. In a steady state population, life expectancy at birth times the rate of births gives you the number of people alive, and equally number of people alive divided by average length of life gives you the number (or rather, rate) of individuals born, which is what you need if you want to know how many individuals have ever been born. The maths gets a bit mor complicated with a dynamic population size, but the principle is the same. The median would be completely useless for this.