To stay ahead of the curve. Advancement. It can do what no other of our current top-of-the-line fighters can do. It regularly whips F-16s and F-15 in air-to-air, even with more experienced pilots in the older jets.
The F-22 is designed for a single purpose--air superiority, and it's excellent at that. The next fighter, the joint strike fighter, is no more than the old F-4 Phantom--the last "joint services" aircraft to be designed. While a fine aircraft (great as a gunboat in air-to-ground, okay in air-to-air role), any "multi-role" aircraft will be pretty good at all roles, but not exceptional at all roles.
Perhaps you feel we should have stuck with the P-38s? After all, they were good enough for WWII. All combat aircraft have a limited lifespan, structually and technology-wise. That's why the army has Abrams instead of Shermans, and why the navy is nuclear instead of oil-burning.