They didn't know any better. They thought they were improving things. They didn't understand that they were polluting the production line until some point in the 70's when the Japanese started to kick their rears. Then they figured out that they didn't have to change everything every year.
I think 60's is more when they last consolidated the engine lineup, heck even before the proliferation of Hemi's and Poly's Chrysler essentially had two engine lines, the 23" flathead and the 25" flathead sixes. The small 6's got put into Plymouth's and Dodges, where as the big 6's went to Desoto and Chrysler and Dodge trucks. Canada was another story though.
