So why is she so bothered that I do not get excited about babies?

There's nothing wrong with you not wanting to pick up the baby. She shouldn't be bothered by it, maybe you just don't know how to interact with babies or you just simply don't feel a bond because it's not your child and that's okay too. I think you have done the right thing by being polite and congratulating her. That's kind of a tough one because you don't want to offend her, but you also want to be honest. I'd just let her know I'm just not that good with holding babies and leave it as that. Maybe she'll drop her issue.