Why are half of the cars in US model by 1918?

That's incorrect. By 1918, a tiny fraction of cars in the US were even *close* to being half of the total number of cars. The automobile industry was still in its infancy. Mass production hadn't reached the levels that would make cars commonplace until much later. Reliable statistics from that era are difficult to come by, but the total number of cars in the US in 1918 was far less than what would be necessary for "half" to be a meaningful statement.