Because science.
edit: after reading so many thoughtful comments, I am changing my view on this matter. If you are sick, and go out in public, please be kind and don't spread your germs everywhere. If this means wearing a mask for you, then God bless you. If you choose for whatever reason to not wear a mask, then God bless you and you should be free to decide that. If you go out in public, you assume risks that there are germs out there. If you are not comfortable with those risks, you have the right to no go out. If you are a business, you should not be required to put up any signs or enforce any such orders. In fact, there shouldn't even be such orders. In other words, let's just go back to the way things were. This may mean imposing a BAN on requiring masks not a ban on the masks themselves. Thanks all for the lively discussion.
Where the hell is OSHA in all this. Some people can wear masks all day like doctors and construction workers because of the environment setup.
Hospitals have forced air systems that circulate air properly in each area of the hospital. And most respirators or n95 masks that construction workers use have a diaphragm that vents expelled air when one breathes.
Children and adults wearing masks sitting stationary in a confined space are literally being slowly poisoned to death by their own co2.