No one here wants measuring standards to be like a condo. Who wants to measure a large house room by room, hall by hall, closet by closet, etc. Don't forget to factor in sloped ceilings on upper levels.
I am not saying that's not the most accurate method for determining LIVING AREA, but in reality, walls are part of living area too. If you have take a large open room, and measure it at, say, 900sqft, should the living area change simply by partitioning that space into 3 or 4 rooms? Walls are part of the house. They contain things that the house needs---electrical, plumbing, HVAC, etc. They are a choice the owner/builder is making on how to best utilize the space within the exterior walls. It does change (decreases) if one does not count walls. I do agree counting exterior brick and other coverings is a bit misleading, BUT what I like about ANSI is A) it is uniform, easily reproducible in theory, and B) any 'error' introduced is relatively quite small compared to the ease of taking the measurements in the first place. Measuring room by room would be a nightmare, IMHO.