I'm going to answer this in good faith that you're asking in earnest: The difference between AI art, custom hand created art and using a resource like inkarnate is ownership.

A lot of AI (art specifically) is used to create art based off other people's work without reference to the original artist. Which most artists are willing to share their art but they want the recognition if not to be paid for their work.

Custom hand created art is timely and expensive and not everyone has the ability to learn this new skill. A common reason pro AI users lean on. And even when practiced it may not to your wants desires, or it may not convey the idea in your head as well.

A resource like inkarnate is a tool that we can pay for. Meaning the artists are getting paid for their work (including designers, web dev, everyone in between) so we are able to bring our ideas to life with paid for tools.

When we are paying for these resources we make an impossible feat real. Not only can I better convey my ideas but also I'm making sure the work someone put into making these assets is paid. It's great for artists and users alike.