Why do the D-series, H-series, and Grand Marshall not have options for rear drums?

I was going to see how drums and discs would affect vehicle handling compared to a disc only setup. But when I tried looking for them, I realized that there wasn't an option for rear drums at all on any axle on the D series, H series, or one for the Grand Marshall. It just doesn't see era correct for American vehicles like this to not have the option.

 

Because they're Gavrils
Drum brakes just aren't good enough

 

and nor are ohv engines.
or quarterlight windows.

 

The Covet could do with rear drums too, and maybe the Pessimas.