I don’t necessarily agree with it, but I think many classically trained artists have a problem with modernism because it, more or less, declared war on their artistic methods. Modern art was argued by some to not just be evolutionary, but necessary. You had thinkers, like Adorno, coming out of the Frankfurt school, that basically said if you weren’t making “high-brow” art, your stuff was practically synonymous with the most derivative drivel you could find.
If you hear enough of that crap during art school - which, believe me, you will - it may start to build up some resentment, I guess.
Meanwhile, there is a very real feeling that modern artists use a lot of resources to say very little, as Banksy put it. Intuition tells you that it’s all a very well played game, where people that make it big are intentionally creating the strong illusion of meaning without ever actually saying anything, or having any intent, other than their own betterment. Now I ain’t saying this is true in all cases, but the “this is kind of opportunistic bullshit” factor is high in modern art, even stuff that’s “supposed” to be good.
For example, you won’t personally convince me that Damien Hirst somehow deserves to be a famous multi-millionaire, or taken seriously as an artist, for putting dots on a canvas. Yet here we art. Dot man is worth hundreds of times more than your average worker, for his contributions to…it’s hard to say, exactly.
So yeah, people get a little defensive about it. Probably too defensive.