Because we have awesome genetics. Now bow down for your masters!

Well being tall doesn't mean you have good genetics. For most people it's a problem as they age. Look at the Methusaleh Gene; it is only found it short people, and allows them to live for much longer than taller people. Also, taller people generally don't live past 75- they're bodies can't handle maintaining themselves very well after that due to the amount of force the heart must use to push blood through the body, and how much the cells have to work at reproducing constantly.
On the outside, height may seem an advantage, but being short truly is great.

My family all live past 95!
If you don't believe me... why do smaller dogs live so much longer than their bigger counterparts? The 10 year difference in lifespan for them translates to roughly 30 years for humans.