The answer is: he doesn't need to. If he's the POTUS and says everybody should wear masks, that's all the country needs to hear.
Sure there's always knuckleheads, but that's where the state and local authorities come into play. If people aren't publicly shamed into compliance, then local authorities can step in.
You act like the government can't 'order' you to do things. I'm pretty sure that if you decide to go out in public naked or drive a vehicle without a license, or decide to sing a tune inside a library, you'll discover YOU HAVE TO comply with the government's wishes to wear clothes, have a driver's license, and be quiet inside a library.