I see two reasons. Most people that are “left leaning” value both critical thinking and social fairness. AI subverts both of those traits. Firstly by definition it bypasses the “figure it out” stage of learning. The second way is by ignoring long establish laws like copyright to train its models, but also its implementation which sees people lose their jobs

More formally, it’s probably one of the purest forms of capitalism. It’s essentially a slave laborer, with no rights of ability to complain that further concentrates wealth with the wealthy.