This is not because AI tools cannot be used to generate good art! In fact we'll likely see artists adjust their process to take advantage of its strengths. Rather, it's because these tools are used to generate art (whether good, or mediocre) completely automatically, rendering art as a commodity rather than something special to treasure. If you see art as a means to an end then this might seem good. But part of the sheer power of art is in its scarcity; in the rarity encouraging us to take the time to investigate something more deeply, to build up relationships with art and artists, to find meaning and emotion and connection, which takes time and intention.
Imagine an AI-produced endless stream of music that takes all of its cues from your favorite songs and produces high-quality audio tuned exactly to what you like hearing. How long does this remain interesting? How difficult is it to tell this noise from new music which might later shape your tastes? Part of the stagnation this AI revolution represents is an added difficulty of cultural evolution, because instead of there being room for the truly new, we are swamped by imitations of everything that came before.