As a animal why would I need to goto the effort of pleasing the opposite sex if I get all the gratification for free with porn and without the stresses and hassle, of an expensive marriage and nagging?
And that's not to mention the mental or emotional aspects at all. Nor anything about a relationship (if the sex was with a longer term partner).
Porn is about fantasy and serving a short term need/desire. And it's a form of entertainment, fantasy entertainment. No one expects real sex to be like porn, just as no one expects action spy movies like Bourne to be like how intelligence agencies operate. That doesn't mean that regular/real life isn't worth living or doesn't offer something of value that isn't covered by the other thing.