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.

(A) It fits into peoples agenda.

(B) Most people know almost nothing about statistics. Even those who use it on a every day base. Honestly, I'm afraid, I'm not really an exception.

I think if you have to guess between human evilness and stupidity the latter is more likely, right?

Not every poster here calls English his or her first language. If you send me corrections, I very much appreciate it.

"on an everyday basis." <shared in the spirit of learning and mutual respect. Not snark>

Aside from the fact that more than half of the people in the world actually don't know anything about English, what does that have to do with..anything?

I do claim to be at least moderately familiar with the language, and have no idea why you're quoting that line.