Because it is lol
More specifically, atonalism in general. I don't want to go too far into one my trademark anti-atonalism rants but to try to keep it short
Not really, but let's say for a moment that it is complex. Complex does not automatically equate to "good". The complexity must still be highly musical and intelligible, this is why John Williams is so revered in the film music world.
It also true that theoretically any performance can be "expressive", but an expressive violin performance of bad music is still bad music.
The point of music is to sound good at a minimum. What is "good or bad" meets a lot more consistent, objective criteria than modernists want to believe and is recognizable across generations and cultures.
Play a Mozart Sonata and one of Schoenberg's back to back for people who have no interest in "classical" music, in any country in the world, and you will find they prefer the Mozart one every time. Because it is a better piece of music.
The entire development of music as a craft, art and science is rooted in tonality and mastery of it. Atonality is therefore a rejection of this history and thereby music itself. It is thoroughly anti-music.
So I would say it is more than fair to call it "garbage".
Because not everyone may share the same taste or precise aesthetics and not everyone can be pleased does not mean that objective standards of quality do not exist, however. Not everyone agrees that the world is round, but that does not mean that their perception of reality is equally valid.
However, I do notice that most of the people who call John Williams' music "Kitsch" tend to be those producing the low-effort "garbage" I just finished disparaging, and I'm sure that is related.
In regards to Zimmer and minimalism, while it is true what I said previously about complexity, it is also true that you require a certain amount of complexity to be musical at all. This is a big reason for Zimmer's failure. The Dune score is nothing but boring drones and pads.
Why this is all relevant to your thread is that for the better part of 100 years now, corrupt academics and suits have promoted woefully unmusical examples of the orchestra or piano and this has caused laymen to dismiss "classical" music because they do not "get it".