Running a program which generates art is no different from clicking the shutter on a camera, or throwing paint on a canvas randomly (non-repeatable physical process generates the image.
Generating art costs money - electricity to train models and rent/buy the hardware, requires expertise in neural nets, tons of training data from the public domain or privately created, lots of experimenting and fiddling with the parameters, manual curation of the output ...
why should the human who invested all this work not be entitled to the copyright?