Maybe because the parents enjoy holding/hugging the baby, and they assume you will too. It doesn't occur to them that it would make you uncomfortable. I usually worry about not holding it right or even doing something wrong and heaven forbid, while you hold it, it starts crying! Which is embarrassing... I think the parents just want to share their joy with you, and I can understand that, but they often don't understand that not everybody gets a kick out of that.