All the other replies are true, but the main reason is that airplanes fly a constant pressure altitude because it's important performance wise, while flying a

All the other replies are true, but the main reason is that airplanes fly a constant pressure altitude because it's important performance wise, while flying a constant geografical altitude is almost meaning less(except for obstacle avoidance. Pressure altitude is fundamental for the performance of airplanes because you fly in a known estable layer of the atmosphere that keeps your drag, lift and thrust almost constant (there are temperature changes but it takes docens of miles to change a single degree). Also if you select the standart atmosphere you know you are flying the same layer system of all the other airplanes in the air, with a fully autonomous, relatively cheap, and non electrical device. Also it's easy to calibrate on your own if you want to fly in a local airport and have no access to a weather report. It's hard to beat a conventional altimeter.