Because the minor diameter of the barrel is 5.56 mm and the major diameter is 5.69 mm. If the bullet were smaller than that then the propellant would blow past it. They didn't make a 'murican millimetre like they did with the imperial system.
In point of fact Americans have gotten impressive results out of far more complicated metrics than metric. It's not a matter of understanding, it's a matter of pride. And of not having to buy all new tools.
But mild and comfortable is different for different people who are acclimated to different weather.
We need a defined 'mild' temperature. i vote for 70F/21C.
It's a bit chilly for the warm weathered folks and a bit warm for the cold weathered folks. Seems reasonable but I'm open to suggestions.
I'd adjust it to 68/20 just so it lines up with whole numbers in both systems. And on second thought, make it 90 per degree Fahrenheit so any whole F or C value can convert to a whole number.
it needs to be a range, you can't really just have a single point, something like 50f to 70f would be good. Some people like a little below, some people like a little above, the 60s are generally pretty comfy all around though.
We also need to consider clothing as well. Which i do in this case.