Ultimately, it's because the human creators need to eat and take longer to do what they do. If they are uniquely able to create something valued, then we want to afford them some protections so that they can keep doing that value.

For AI works, the effort is trivial (and frankly, the output is very much uninspired, but there are places for that). So there's no connection between human labor hours and the content, and therefore no reason we should prioritize protecting it.

On the stance of whether copyright helps achieve that, if you simply remove copyright without an alternate system, then the creators get nothing at all once a single copy of their work is made available for free. It was bad enough when works had to be printed/manufactured, in the digital context duplicates are perfect and essentially free. Straightforward enough case on perfect duplication, but then it gets rough on "derivative works". You include something created by another person but contribute your own thing and make it new, well, you clearly derived some value from the inspiring works but clearly also created your own value, and that's so subjective. Finally you have the terms of copyright, which seem crazy long, and could stand to be shortened.