"Since when can a governing body tell us what we can and cannot lawfully do?" All the time. Think of this as the equivalent of a speed limit, it exists because people have poor judgement about levels of risk. You might feel you can go 100 mph, but if the road is designed for 60 mph you are putting yourself as well as others at risk, sure you might just crash into a tree and only hurt yourself, but you might crash into someone else and hurt them or their family. That is what the lock down is about, everyone else. You might not care about your own health figuring you can beat it, and maybe you can but while you are sick and are asymptomatic, or have mild symptoms you can spread the virus, putting other people at risk, the same way you would put them at risk carelessly driving / or excessively speeding.
37 million Americans live with a chronic lung disease
70 million adults in U.S. are obese
34.2 million Americans have diabetes.
Just these 3 conditions alone equate to HALF the population of the US, and that's not counting the elderly. These are the people most at risk of death, but people with no known conditions have also died from this.
On other thing I want to point out, an actual death rate is determined on the conclusion of an infection, ie. Recovered vs Dead, not Infected Vs Dead Recovered vs dead in the US right now 49,000 deaths, 85,000 recoveries. that's 36% casualty rate for anyone who was able to get tested and was positive, and while there may be many more infections that are asymptomatic or never get bad enough to get tested it is still not looking good.