Why is it exciting to learn that someone else's baby is either a girl or a boy? It's going to be either one or the other, isn't it? There's an implication that there's somehow a third option that would be less exciting.
Well, the baby could be intersex. The gender reveal party adds a new layer of horror to this; now there's an added motivation to have healthy body parts cut off and assign feminine gender, if that's what was "revealed". 
Oh, or the scan image could have been interpreted the wrong way by a doctor - in which case you'd have to tell everyone the actual sex of your baby all over again. (Or trans the poor kid ... I fear that will actually become possible in the future, even for babies)
Gender reveal parties are somewhere in the tradition of parties thrown after birth, where in some cultures the party for a boy is blatantly twice as long/large/expensive as that for a girl.
Not only do I not get them, I consider them actively harmful.
It's not like the reveal is "surprise! It's a dinaosaur!'
I wouldn't be surprised if "dinosaur" were one of those new sparklegenders people "identify as".
Of course, the sex is either girl or boy, but it is called a gender reveal party ...