I would say there's probably a lot of reasons people are doing less with their free time thay relate more to the lack of free time and/or money than just that they're satisfied watching it on the screen (though I recognize that plays a part).
if you're going to say porn is useless specifically bc you can just go have a sex, though, you might as well take masturbation off the table, too.
sometimes I don't want to get laid, I just want to get off. sometimes I'm tired, or don't have time, or my partner is tired, or on tour for three months, or whatever. I CAN do it without porn, but it can be enjoyable with it.
as for the ease of getting laid: some people have physical disabilities, or or cognitive or social impairments that make connecting with people difficult. maybe they're too self-conscious about their genitals. maybe they are afraid of STIs. maybe they have an STI that makes finding sexual partners difficult. maybe they're hella gay living in in a red state with their conservative parents & getting out there risks their life or job or home.