Likely training and running cost.
Most AI art generators are run on single GPU and can be trained with some top-of-the-line consumer hardware. Expensive but accessible.
A full blown LLM like ChatGPT is literally the cost of a small startup to build and trained. Running it is near impossible without cards like A100 which alone costs more than a full enthusiast grade PC.
Maybe eventually they will distill and optimize the models so that we can fit these things on a PC, then laptop, and then phones. But for now it is exclusively the domain of big tech.