One reason for this is that in larger trees, the rings themselves play less of a role in causing warping. Another is that many trees that make it to 50 years in a modern forest wouldn't have made it to 400 years if left alone. Old-growth forests have much greater spacing between trunks as the overgrowth trees that were successful outcompeted their neighbors; to have survived 400 years indicates a tree that was given superior placement and nutrition.
Finally, when trees are much older, they are much larger. This means that relatively more of the wood comes from the center of the tree, where there are fewer knots, voids, inclusions, etc. The moisture bearing part of the tree is on the outside, and in many species is unusable, so you have a much higher uniformity and yield in board feet from a bigger tree.
That said, some things where higher quality back then. For example modern fast growth wood is far lower quality let alone things like particle board.