Because that's the difference (now I get a little technical): in classical flu, besides that it infects much less population over several months, cases are complicated less frequently: only when the virus has destroyed the protective barriers of our airways and as such it allows bacteria (which normally resident in the upper airways) to invade the bronchi and lungs, causing a more serious disease. Covid 19 causes a banal flu in many young people, but in many elderly people (and not only) a real SARS because it invades the alveoli of the lungs directly, and it infects them making them unable to perform their function. The resulting respiratory failure is often serious and after a few days of hospitalization, the simple oxygen that can be administered in a ward may not be enough. Sorry, but to me, as a doctor, it's not reassuring that the most serious are mainly elderly people with other pathologies. The elderly population is the most represented in our country and it is difficult to find someone who, above 65 years of age, does not take at least a pill for high blood pressure or diabetes.
I can also assure you that when you see young people who end up intubated in the ICU, pronated or worse, in ECMO (a machine for the worst cases, which extracts the blood, re-oxygenates it and returns it to the body, waiting for the lungs to hopefully heal), all this confidence for your young age goes away. And while there are still people on social media who boast of not being afraid by ignoring the recommendations, protesting that their normal lifestyle habits have "temporarily" halted, the epidemiological disaster is taking place. And there are no more surgeons, urologists, orthopedists, we are only doctors who suddenly become part of a single team to face this tsunami that has overwhelmed us.