Tbh I think this is part of our basic sexual instinct, as it holds true when finding women irl as well. (also yes I do the same thing)
Firstly, finding the perfect woman for mating is something that goes back to our days as cavemen, obviously. So why wouldn't our brains be wired to essentially do the same thing when it comes to porn? Searching and searching for the best hips, breast size, legs, pretty faces, and even situation that turns us on the most - since that is the most basic instinct when it comes to sex.
Secondly it's similar to sex in a way too, as the entire process from start to finish is stimulating to us sexually. Not just the orgasm. We enjoy the chase, the seduction, the foreplay that turns us on and feels good to our brains and bodies, and the sex itself which can, all right up until the orgasm itself, still be considered along the same lines as searching for the perfect porn to fap to at any given moment.
Think of porn as a woman, you do all kinds of crazy things with her and try all sorts of different positions and things that feel good right up until the moment you cum. It's kind of that simple