Except that historically the reason not to max out memory on purchase was often cost. (Sometimes availability.)
By the time prices of memory fall sufficiently, or you have saved up enough!, you might be halfway through the life of the computer. Taking out the limited memory and replacing it by higher capacity still often seemed expensive. And you have the spare DIMMs which were, when bought, quite expensive. And you will likely only use that extra memory capacity for the second half of the computer's life.
By the time you actually replaced the computer, even the enhanced memory capacity tended to look quite limited, speeds improved, even form factors and allowed combinations might have changed. So limited options for re-use.