Earlier quoted context omitted.
The article hits on this, suggesting that they are viewed as communal property since they're so abundant, cheap and frequently forgotten. Umbrellas are also an on demand item, so can see how this could evolve as a sort of community solution to the rain problem.
What is the rain problem? I live in the Pacific Northwest where it rains for 9 months out of the year. Our solution is to not use umbrellas at all. Everyone has a waterproof rain jacket.
PNW rain season is winter, summer is fairly dry.
Japan is the opposite, and quite a bit warmer than PNW (at least at Tokyo’s latitude). Sweating is a problem as is in summer clothes, rain jackets would be detrimental to health.