There is no doubt that trucks are better than horses
IF
You can actually build the number of trucks needed.
You can supply the trucks with fuel.
You can maintain the trucks.
You can 'crew' the trucks.
The advantages of trucks had been figured out by more than a few armies
before WW I. British for one paid a 'subsidy' to commercial operators to use suitable trucks to be 'mobilized" in event of war. Of course if you mobilize too many commercial trucks your domestic transportation system craps out.
Military men had figured out the fodder problem back in Napoleons time (if not before) you can only advance so far before the horses are eating the entire payload of the wagons.
Germany's problem with motorization were many.
#1 was they simply didn't have the factories to produce the needed vehicles.
This page on Wiki is fairly illuminating as far as the US goes.
U.S. Automobile Production Figures - Wikipedia, the free encyclopedia
In 1937 Plymouth was the #3 car maker in the United States and built in one year 95% of the
total number of military trucks produced by
ALL of the Axis nations for the
entire war. Granted the Plymouths were NOT military trucks but it gives perspective.
You can wave magic wands about and talk about funding or money spent on the XXX program but it takes several years to build and equip factories and train workers. You can't build trucks very well in shipyards. Each nation only had machine tool industries of certain sizes and it was the machine tool industry that limited the number and types of factories that could be built. You can put up all the empty buildings you want ( subject to structural steel and concrete limits) but you need machinery to put in them.
Yes you can expand the machine tool and factory equipment industry but now we are talking about a 2 or 3 generation factory building scheme. Lathe company "A" only delivers 75% of it's production to truck company "D" while 25% of it's production goes to it's own new expansion plant (Plant "B") which will
start delivering machine tools to truck plant "G" in two years time? and so on.
Horse traction was also much more complicated than 6 horses per gun team or wagon. 6 horses was about the
minimum pre-war for a team. 8 horses were preferred for even medium artillery and the bigger weapons went for more horses or broke the the gun into two loads even with 8 or 12 horse teams.
You then had at least 3 categories of horses. Calvary horses were almost useless for load towing. The big draft horses were actually not able to stand up to bad weather as well as some smaller breeds.
A lot of European countries between the wars were a strange mix of industry and agriculture to modern eyes. There were very heavily industrialized regions and some regions with little industry. In the heavy industrialized areas the vast majority of workers either walked/biked to work or used street cars/trams/buses. Factory workers and their families rarely had cars. The rural/agricultural areas also rarely had money for cars/trucks. The plow horses often pulled the wagon to market.
There were farm trucks and tractors but they were much less common than they would be even in the 1950s/60s.
The Germans had also learned a lesson in WW I when they mobilized
TOO MANY horses form the agricultural sector and helped cause the food shortages of WW I on the home front.