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Usually artists also don’t police and get so angry about other people’s self expression.
The NYC gallery scene started bestowing art-ness upon people who became known for the work they painted on the city's walls around... exactly forty years ago, I looked up Kieth Haring as a random example and galleries started showing his work in 1983.
Why? I would assume "what is art?" is a subject on which much time is spent in art school.