[craftinator]

Because I've used them. The most complicated firearm the average grunt will use is an M249, which has fewer moving parts than many handguns. Production inertia and poor accuracy keep the AK series out of the US armed forces, along with a preference to 5.56 over 7.62. Basic maintenance for an M16 is nearly identical for an AK-47, the M16 is lighter and has smaller/lighter ammunition, and has better mid-range and long-range accuracy. So let's say that there is a very slow curve to complexity vs convenience up to a certain point. But we have been using the M16 since the 60's; plenty of newer, more complex rifles have come out in that time, many with better range, accuracy, stopping power, cyclic rate, etc... But we haven't moved away from a simple, fairly reliable rifle.