people who complain about modern art almost always know essentially nothing about art. Usually heavy overlap with those that complain about anything that is different from what they like/grew up with

Because the public never gets the opportunity to be exposed to good art, whether classic or abstract or whatever style. Most what's called "modern art" in the style of early 20th century like Picasso or later Pollock is low quality sewage, but that's what's being fed to the public. That's what everybody grew up with and everybody hates.

The public is underexposed to beautiful and interesting art, but who is to blame?