They pay for infertility treatments because that issue was successfully pushed by lobbyists and persons with a monetary interest (doctors, infertility centers, etc.).
Yes, infertility is a medical issue. So is not being able to see without vision correction. Yet, most insurance policies will not cover corrective laser surgery for vision, classifying it as "vanity" surgery. Instead, people have to purchase eyeglasses every year or two as vision changes (something that the surgery would eliminate). In the long run, the expense of eyeglasses greatly exceeds the cost of the laser surgery, making this decision a puzzler when looking at the bottom line.
No one ever said our current medical insurance system made sense. Actually, I believe that's why we're now stuck with something even worse.
I don't see a problem in covering infertility treatments if it is capped at a certain amount or number of efforts, much like current vision insurance. Some things just can't be guaranteed in life and life is not fair. Part of learning to deal with life is accepting and overcoming our personal limitations. Sometimes, we just can't have what we want.