Because people are interested in whether they, personally, could find a way to live longer. And so if you find places where you have bubbles of people who seem to cross a threshhold not really crossed in other places, you might tell yourself "maybe there's something special there".

The median life expectancy means you're only in the top half! But well... simply being well off means you're going to be more likely to hit above average there.

Extreme agers are rare enough that they're highly unlikely to show up in median statistics. If there are people living exceptionally long, they're still a small proportion. You might get some mileage from percentile analysis (95%ile, 99%ile), and there may be robust research methodology along those lines (I'm not tapped into that).

Which actually does leverage your concept somewhat (increase the sample size to include more members, and reduce extreme outliers). There is also plenty of analysis which does benefit by median (vs. mean or maximum) analysis, or by looking closely at observed distributions and outlier characteristics.

Median life expectancy is affected by a much larger number of variables, and is also susceptible to just as much (if not more) number fudging and statisitical nudging. Maybe the country also has a large subset of obese alcoholics for example. If someone lives to be over 100, that in and of itself is a pretty valuable data point

because that's generally not enough to separate lifestyle from genetics and environment.