Because over time, those most likely to engage us in combat, or at least test the ability of our military assets, adapt and start to render current assets obsolete.
http://www.airforce-magazine.com/Mag...0307force.aspx
One way to keep the price tag down on military assets in, for example, the Air Force and Naval aviation is to continue to deploy remote controlled drones. The drones are perfect for the search and destroy missions that have are part of our anti-terror efforts. However, over time, the enemy will likely adapt to these assets and tactics, and well need to go find another weapon to press the advantage against the enemy.
The other reason is that mechanical equipment is built with an intended life expectancy. After that point, maintenance becomes less cost effective, and replacement is required.