It's cheaper to do so for many hunters. Plenty of people don't have access to quality hunting land. By the time they bought or leased land, ran food plots, bought stands etc. it would be a small fortune. It's cheaper and easier to pay to hunt many times.
Public land hunts are available, but you run into spending tons of time and facing over-hunted areas. I think it many cases it's cheaper to pay an outfitter than it is to buy or lease land.