Why is everyone's opinion equal? When building a bridge, is the opinion of a civil engineer equal to that of a cook? No, clearly not - but we only recognize this because the effects of building faulty bridges are obvious and immediately noticeable, whereas the effects of cultural ignorance are far more subtle and wide-reaching.
The fields of art and aesthetics have a long, complex history and have been studied and written upon by thousands of intelligent scholars, designers, artists, philosophers, and religious figures. Unfortunately, the default opinion among society today (which you yourself are echoing) is just a sort of ignorant relativism - everyone's opinion on art (and anything else that isn't 'practical') has the same value and the topic is fundamentally just a popularity contest.
Worse yet, this opinion wasn't arrived at by carefully considering all the approaches to art and choosing subjectivity as the best option, which it may actually be (but that isn't my point.) Instead, it was arrived at by simple ignorance.