Someone cooked up a model showing that the embraer super tucano would have been more effective in the CAS mission in afghanistan than some of the high power jets we used, in addition to 10x cheaper.
I would love to see such airframes flown, they would be much more effective in many roles than "super-airframes" like the F-35. But that's not the US military philosophy. We fly the best and most expensive, even if it's 10x as expensive as an airframe that's 75% as effective. It's a calculation that government debt is cheaper than having to face the public over killed and captured soldiers.
Also have heard that ground commanders hate the high-altitude planes for CAS because they're less likely to correctly discriminate the situtation on the ground before unloading.
To your point -- yes, we don't operate AC208; I think iraq is the only one who does. (And if the sight of a hellfire stapled to the wing of a small turbopro doesn't make you want to run and hide, I don't know what will). But we also don't operate the F-35 in the CAS mission. neither plane is a good example of a CAS plane.
Even considering that hypothetical scenario, there's also the cost of training everyone throughout the logistics chain and required to get the plane operational and in the air.
If you lose a peasant, you lose a peasant. If you lose a knight, you also lose the squire, the page, etc etc etc.
The squire can help some new guy with his helmet, etc (I think this is considerably more true for modern military than for knights...).