1st you ask why, but then you give restrictions to your question...
its the challenge, once you've flown everything there is in the inventory, no more ACM's to learn, you grab the best challenging plane and try to take ACM even farther buy flyin what is the hardest vs. the easiest...
ex:
grab a 202 and go fight LA7's, the lala will have a Golly-geen hard time killing you, and you will take all your ammo just to hurt em:D
my hardest ride was always the F4U-D, under 10k vs. spits niks and lala's...
oh and always loved turn fightin spits in Typhoon...