Why are girls dad's favorite and boys mom's?

I can’t speak for other people, but not in my personal experience. I don’t have a favourite. I have three unique children (two girls and a boy) that I love equally. They are each so very different, but equally special to me.

Oedipus and Electra.

Plus, the child of the opposite sex is more likely to create a similar dynamic to you and your spouse. They are more likely to behave like them and even look like them.

It is a sexual thing, really.

I only had boys so I have no basis for comparison.

I don’t really know, but I will say that there’s very few little boys I like (too aggressive, too stupid), but I love my daughter, my nieces, and my daughter’s girlfriends. Maybe I’m just wired that way.

I’m in the same camp as @Earthbound_Misfit & any other parent worthy of the name, no favourites, I worship the ground my son & daughter walk on just the same.
Couldn’t possibly have it any other way.

Historically, in many cultures, boys have always been their parents’ favourites. Boys can carry the family name (thus earning parents’ inheritance), are expected to be the money maker for the family, and simply because having more men will provide more protection and safetiness for the related family. I wouldn’t be surprised to see that boys are still preferred in many of today’s traditional societies, just take a look at Middle Eastern and Chinese society for example.