Because it is very evident that every mask prevents you from breathing freely and therefore from receiving less oxygen and, conversely, the carbon dioxide that you exhale accumulates in the mask, so the result is that you have a lower energy intake and then logically, the fact of not being able to expel a greater quantity of air accumulates over the course of the day many bacteria in your body that your body wanted to eliminate and so you breathe them in again and again.
The result: these masks are a kind of modern torture put on for the fear of getting sick instead of leaving the human body to fight the thousands of viruses with our natural protection that must defend itself every day from natural aggression and if you prevent this, your natural protection decreases more and more and your antibodies are considerably weakened so what you consider a salvation turns into a defeat for the work of your antibodies.
It's logical that doctors and nurses wear masks in operating rooms because the human body undergoes an operation and is consequently deprived of active antibody protection, being busy trying to survive, and is therefore greatly weakened.
But in practical life, masks are more harmful than beneficial unless a person already has very low levels of antibody protection. In this case, it could theoretically be beneficial for short periods, but I doubt it will be beneficial in practice in the long run.