Because ISFPs are The Artists of MBTI, so people think all artists are ISFPs just because of the label. By which logic ESTPs are the doers, so no one else really does things, and INTPs are the thinkers, and really no one but them has ever had a thought. Makes complete sense to me.
Besides, INTJs are stereotypically all evil anyway. Evil people don’t like art.
MBTI stereotypes are stupid basically. Even people who have an understanding of the functions are sometimes so quick to fall into this generalized behavior assumptions, forgetting A) that Ni can be a very creative function, as can any perceiving function and B) that some personality characteristics aren’t accounted for in the basic MBTI theory. Creativity is one of these traits, as are maturity and kindness. These are things you can’t associate with one type because everyone has the potential for them. Talents/Interests also have nothing to do with MBTI. Some are more common in certain types, but correlation does not equal causation.
Don’t mind what these people say; their words neither invalidates your type nor your interests.