“Why don’t they make M-16s in 7.62?”
Ammunition supply train issues. 308 is bigger and heavier than 223. A soldier can carry more 223 ammo.
Personally I prefer the 308 but I don’t have to carry a battle load.
Nothing like 5-6 loaded 20 round magazines of 7.62 on your waist to see the advantage of the 5.56 for the average grunt.