Im betting it has a lot to do with construction costs, especially with chairs. Chairs are very expensive and difficult piece of work with tons of pieces that ne

Im betting it has a lot to do with construction costs, especially with chairs. Chairs are very expensive and difficult piece of work with tons of pieces that need to take serious abuse, more legs means more pieces. Also if you have even the smallest imperfection in angles or squareness on a 4 legged piece of furniture it is immediately obvious. Joinery is what takes the most man hours, so hose stools for example have 24 mortises each, add another leg and you got 32 mortises, and a more expensive piece with more points of failure. Another thing to consider, the floor. How flat would most floors be at the time? 4 legged chairs would almost always be rocking on 3 legs, causing them to fail and break more often. Shimming a short leg only works if it doesn't move or if the floor is almost perfectly flat.