Because they are trying to be cute. It makes me throwup in my mouth a little when I hear that.
Yes, both parents contribute to the conception, but this is just too cutsey for me and it makes me hate men who try and please women by acting like they are women too.
The use of cutesy phrases such as "we're expecting" of demasculinating phrases, sickens me. That's what I meant with the photo I posted, not anything about pregnant women having morning sickness.