The cabin pressure in aircraft does decrease as it gains altitude, that's why your ears still pop when your plane climbs or descends. This is because if the cabin pressure remained at a level you find on the ground then it would place a lot more stress (net force) on the airframe due to the bigger difference with the external air pressure at 30-odd thousand feet, so lowering the cabin pressure as much as possible whilst remaining safe for passengers means that planes can be made a light as possible.
One big source of the airframe's weakness comes from the windows that they have, airlines would much prefer it if passengers had no windows at all, making for a much more sturdy airframe, reduced weight, reduced costs etc etc. But passengers would probably feel quite uneasy about not being able to see outside. So lowering cabin pressure is all about money!