If I buy a pencil and draw a copyrighted character, is that copyright infringement? No. If I sell that drawing? Yes.

Midjourney and other AI companies are not selling you the drawing, they're selling you the pencil. What you do with that pencil and the drawings you create with that pencil is on you and you alone.

AI inputs are not outputs. Copyright exists at the output. You can't sue someone for reading every book in the library unless they try to sell one of them as their own.