Except that they make AR uppers for just that caliber now...
Do you have a link to an AR in 7.62X39? I know they have a couple in 308 (7.62X54). The 308 round and the 223 are both more expensive than the stuff I feed the AK. I considered the AR when I got my AK, but when I put the 223 round next to the 7.62X39, that was it.