> CDC specifically states that transmission occurs with prolonged (15+ minutes) contact in close quarters (less than six feet).
No, that's not correct at all. But if I may take a guess at what you meant to write..
Those are guidelines based on estimated inflection points where more time or less distance increases the likelihood of transmission.
They are presumed to be good-faith estimates, but they are based on difficult-to-collect data. It's probably reasonable to draw the conclusion that, if fully observed, the guidelines would drive R0 below 1.0 and the trendline would follow.
But that doesn't mean that no transmission occurs in other situations. People sometimes sneeze unexpectedly. It takes a fraction of a second to put on the mask that prevents that event from becoming risky, or at least uncomfortable. Cheap insurance.
It's also important to realize that "walking by someone while outside" can take many forms, so it's not clear that we're all talking about the same thing. Jogging on a narrow path is very different from strolling on a beach.
Different measures are appropriate for different cases, but it's never harmful to use the mask, it's a reasonable habit to get into, and it makes people around you more comfortable. What's the trouble?