[Storebror]

If F-15 is the best world fighter, undefeated by none, why spend a lot of money developping F-22?

That's pretty simple:
It's the fear of people from outer mongolia to develop something which greatly outclasses the F-15.
Rumors are that they've had those plans in 45 already, maybe stolen from the germans (or just downloaded them from an argentine website).

Best regards - Mike