It's just the way socializing with other people works. You pretend to be interested in what's important to them and they do the same for you. You exchange pleasant lies in order to grow closer and gain the benefits that come from cooperating with another human being. When you refuse to participate, people aren't really shocked that you're not interested in the subject, but rather, that you would be so rude to someone else. It's for exactly that reason that you can't call a fat person fat. People don't get mad at you because it's untrue, they get mad at you because it makes the other person feel bad.

In short, you don't have to be interested in babies (or cars, new jobs, sports teams, health issues, vacations, favorite coffees), and no one honestly expects you to be. You do, however, have to feign enough enthusiasm to make people feel good about themselves if you want to cultivate relationships. Blame society or human nature for that, I guess, but not babies. They're just another "thing."