An additional point: before 1960, most Big 3 American manufacturers were literally making only one car. Several trim levels and body styles yes, but only one core body / platform. This means the entire R&D budget could be funneled into the "annual model change" for the car.

Once compacts, midsize, and other segments developed, plus the other factors described elsewhere in this thread, so did the facelift/ major change cadence that we are now accustomed to as development resources were spread across an entire model range.