The oldest demographic isn't the only one at risk, though. Obesity, diabetes, autoimmune disorders, and on and on. Over 1/3 of the entire nation is at an elevated level of risk for COVID complications. So over 1/3 have to shelter in place while those who are healthy get to walk around unabated and continue to give the virus hosts, prolonging the situation and further increasing the odds that the at-risk population become infected despite their precautions? Stay-at-home and mask orders aren't quarantine. They are mitigation tactics that serve several purposes, the most important being the decrease in infection rate to a manageable level that won't cause a massive spike that hospitals are overwhelmed by.
I leave my home fairly often for groceries, going to the park, Home Depot, etc. But I keep out of bars, restaurants, and basically anywhere that I don't need to be and I wear a mask if I'm going to be near people and I always wear a mask indoors. This isn't an all or nothing deal, here. People aren't being asked to lock themselves in their home. They're just being asked to stop unnecessarily exposing themselves to possible infection by having parties, participating in indoor recreational activities, and working from home if at all possible. The fact that some people can't come to grips with that is insane to me. We could have been close to being over this, by now, but now we are closer to a vaccine than a community-led effort to eliminate the virus.