Because the flu and COVID are not on the same level, neither in terms of infectiousness, nor in terms of serious complications or death. The worst flu outbreak in the last 20 years resulted in about 65,000 deaths in the US. We've lost over 630,000 people to COVID in the last 18 months and a similar drastic disparity in hospitalizations between COVID and the flu exist.
I dismiss it because I do understand actual science. I also understand that we are neither a libertarian nor anarchist society. When people's "liberty" puts other people at risk and overwhelms our hospitals, we as a society have a right to say that there are limits to that liberty. We can require a vaccine for the same reason we can tell you that you have to carry liability insurance on your car to drive, or that you can't drive 130 mph on the interstate. Or that your children, absent a documented, verified medical condition, have to be vaccinated to attend public schools.