Why do so many PCB-designers use non-metric units?
As with a lot of electrical engineering, the answer is tradition.

For one: it simply doesn't matter which system is used. The only advantage metric offers at all is that units of the same measurement factor by 10^N. Well, an inch is 10^3 mills, so that point is taken care of. Other than that, you never need to convert between units for anything. Copper weights stay in ounces, track sizes and board dimensions are all in mills or inches (which, again is scaling by a factor of 10^N).

Two: On your AWG point, that also doesn't matter because wires are manufactured in discrete sizes. You don't need a continuous unit like mm^2.

Three: Most of this was pioneered in the US, where imperial units were and still are dominant. It would take a ludicrous amount of effort for very little benefit to the people who work on them. All our standards etc. are in imperial. If we converted them, then what? It's all being produced and interpreted by American engineers who are used to imperial anyway, so what good is that?

Four: Any complex calculations are not done by hand, they're simulated in software so again, there's no need to ever convert between anything.