it's easy to forget that most people act with consideration and respect towards each other most of the time, whether or not there is a credible threat of punishment. we tend to notice bad behavior more in our day-to-day lives precisely because it is not the default. you never notice all the dog walkers that dutifully clean up after their pets, but you'll surely remember that one piece of dog shit you stepped in on the way to work. the news almost exclusively covers outliers.
in my state, we have mandatory shelter-in-place under threat of fines/imprisonment, but the police don't really have any way of knowing whether you are violating the order. people mostly follow it anyway, because they are convinced of the severity of the issue.
I think consideration here is extremely subjective. If what you say is even true, that most people are considerate, what defines considerate here? People think they should leave their shopping carts wherever since that's the job of the cart attendant. Are they considerate because without them, that person may not have a job since all the carts are just pushed back to the store?
unfortunately, we don't have a good system to support barbers, who would prefer not to risk catching/spreading the virus but need the money, or barbershop owners, who would prefer to stay closed but need to pay rent. so I do actually support the lockdown as a somewhat dirty hack to trigger benefits for these people.
Why is it that for all of history this hasn't been an issue for infectious diseases until now?
All perfectly legal.
How do you balance that with another person's right to live? If you as a free person decide "F it, I don't want to stay cooped up despite my positive diagnosis." Is that fair to someone you then infect out in public?