Because most house cats are actually unable to live that well in the wild. Though they have the hunting instincts natural to their species, it actually requires some experience (usually learned during kittenhood and young adulthood) to put those instincts to best use; most cats cannot hunt well enough to sustain them long enough to gain the necessary skills.
Obviously cats which have lived outdoors - farmcats or free-roaming pets - have a better chance at survival than indoor-only house-cats, but even free-roaming pets fare poorly compared to feral animals that have lived their lives entirely on their own wits.
Cats also aren't safe from predation; owls and hawks won't pass them by when they are young and even after they achieve full size, coyotes and foxes are known to go after them. Housecats also have a poorer chance at finding good shelter in bad weather, and usually have to fight feral cats for territory, which further hampers their chances.
Putting a pampered house cat into the wild and expecting it to survive because "it's a cat" is thoughtless; it will likely live a very short life before dying miserably in a ditch somewhere.
Whether that option is better than euthanizing the animal is debatable and probably dependent on the beliefs of the owner. The proper option is to spend the time and money to get the animal properly placed, but dumping the feline is cruel and shouldn't even be a consideration.
Sad caveat: the ferals and strays in my town are very well cared for by well-meaning locals; many are fed daily, and have had their shots and been neutered. Unfortunately, this care just encourages more people to dump the animals in our neighborhood.