IIRC there was pretty stiff resistance inside US Army, mainly Cavalry Branch of it. It was even mentioned pretty recently on the forums.
This is why the US doesn't start with Motorized. Motorized is not trucks, it is a doctrine. And in 1936 the US Army was under-funded, under-equipped, and still mired in WW1 doctrine. The Cavalry arm fought a rear guard action up into 1941 to keep their horses.
And while trucks were essential for support companies, the Army didn't have the funds for them in 1936 to fill out all their ToE. Congress had cut military spending to the bone during the Depression. But they did have horses. So I can see where PDS decided no trucks at start for the US.
As far as the Marines, they really didn't begin serious research into modern beach assault doctrine until post 1936. The Marines existed in 1936, but were dispersed into penny packet deployments around the world, stuck guarding embassies and legations, or serving on board ships. The last time they deployed in division formation was 1918 until WW2.