When you were saying how large the gaps were, I was wondering if wood shrinkage along with everything loosening up might be part of the 'WHY'.
Even all metal cars have expansion and contraction. My Auto Body Instructor told us that from a cold morning to a hot afternoon a car could easily 'grow' 1/4".
Long story short: A friend who had grown up in Southern California had a new '76 Z28 and an overhead power cable was dropped on it while it was parked here in Walla Walla and needed a complete paint job after the repairs. He insisted that the top coat be lacquer no matter how much I told him why not he insisted.
Less then two years ago that lacquer finish was cracked and hazed due to this area usually getting sub zero winters and 105+ summers and his car was a daily driver.
Years later when he had it repainted it wasn't in lacquer--:w