Because keeping six feet away from everyone for months on end is unsustainable
No, keeping the economy shut down for months on end is unsustainable, as is printing up more new money to pay for it.
and you're seeing it everywhere.
It's a personal choice and a personal responsibility. If people choose to close that distance, then they accept the consequences of their choice.
The mask is a shortcut.
Also a personal choice. If a person chooses not to wear a mask, then they accept the consequences of his/her choice. No different than the 6-ft rule.
It reduces the necessary social distancing to about three feet, close enough to shake a hand and a distance that most people rarely ever get closer unless they're in a crowded club or are hugging.
Shaking hands and hugging are also personal choices. If a person chooses to shake another person's hand and then touches his/her own face while adjusting his/her own mask, then that person accepts the consequence of their action. And no government directive issued by any governor/mayor/etc. without legislative approval is going to change that.
Personally, I happen to be an adult who is instilled with a sense of personal responsibility. And I am not about to embrace some fascist reactionary impulsive mandate because someone else isn't being controlled like I want them to be controlled. And if the Euro-solution is so effective, then why do countries like France, Belgium, and the UK have higher death rates than the US?
You want an American to wear a mask? Then ask nicely. You would be surprised what that will get you. But order an American to wear one? You may end up with a boatload of masks tossed into the harbor.