My theory is that macOS's flat design made it unnecessary to use the native UI toolkit, because any fool can make equally bland gray flat buttons using CSS. It

My theory is that macOS's flat design made it unnecessary to use the native UI toolkit, because any fool can make equally bland gray flat buttons using CSS. It used to be impossible to beat pixel-perfect obsessively polished Aqua UI. You just had to use native Cocoa, because everything else looked like a cheap knock-off. Now you can slap border-radius + box-shadow and your widgets will look the same or better than Apple's current confused half-iOS design.