IMO this is all really dependent on the individual; its hard to evaluate how someone deals with stress or how difficult their workload and life is off the bike.
I only have a wife, work from home 2 days a week, and have a sedentary job that is stress free (since my stuff never goes down
). I also know how to manage what little stress I have (whining on forums helps, though it annoys everyone). Hypothetically, I could train 20+ hours a week (I'm at 15ish atm) if my body could handle that. That's a huge contrast from a person with kids, a tight budget, an overbearing boss, a stress job with physical demands and long hours, etc.
What the average USAC racer needs probably isn't that high. The top end of 45+ Masters racers (at least the ones that post on Strava) only average about 12 hours a week.