Because there's a lot of medical evidence from around about 100 years to back that up. And flu is also noticably different some years. Last year's flu was relatively mild, this year's by all accounts is somewhat worse unfortunately. I don't have personal knowledge as yes I don't test myself but...
But unfortunately although we can buy kits, most people will not pay for them. Covid is not a cold, and can be extremely nasty, but I've only had it once but have tested probably hundreds of times for work. Even if I had only tested with a symptom or two, I would have spent a fortune and I could...
Covid is out there whether we like it or not. Most people don't have access to test kits. Just as others have said, people who feel awful will not go to work/games/shopping etc but if symptoms seem mild I've no doubt almost everyone will carry on regardless. I am unsure why you are picking on...