Why US doctors say that there is no need for those healthy to wear masks, but in Asia, doctors recommend everyone wear masks for COVID-19?

It makes people feel better. 

Because there are no masks available and they don't want panic.

In Asia, they recommend everyone wear masks because of pollution. The only connection to Covid-19 is that pre-existing respiratory problems make you more susceptible to ANY respiratory virus.

I think maybe you are confusing the wearing of masks for such seasonal nuisances as hay fever and so on, for some sort of public health measure. In the case of Japan, denuded deciduous forests were regrown after the war with conifers that torment the afflicted this time of year.

Yeah, such an abundance of caution seems puzzling in an area of the world where these pandemics originate because dishes such as “wild bat stir fry” are considered haute cuisine ...

Different logic in medical care. The idea in the USA you wear a mask to keep yourself from catching something. It is all about ME ME ME. In Asia the idea is so you do not infect someone else. It is all about the COMMUNITY COMMUNITY COMMUNITY.

Increases death kill.

Most masks allow air to freely pass in and out at parts of their edges. Plus, people NEVER stop fooling with them...WITH THEIR FINGERS. That's not good.

The N95 masks are the MINIMUM required to be useful. And the wearer must fool with them as little as possible. See: https://en.wikipedia.org/wiki/NIOSH_air_filtration...

The practice of wearing masks in Asia began as a means for those who might be sick to prevent spread of their illness to others... it was not something done by healthy people to protect themselves...

This is just another subtle indication of significant differences between Asian and American culture.  Most Asian cultures stress the individual's responsibility to society while Americans tend to be more focused on the self.

That’s because people are walking around without knowing they have the virus.