Barbies are fun. But that's not the point. Marketing is about identity at least as much as it is about the product. You look at beer adverts, how many of them directly bear on the beer?
http://youtu.be/MX145Tu4MHY
That could be any beer, or a magic potion, you know nothing about it.
Or shower gel
http://youtu.be/DVm00UBdS0Y
Or cars
http://youtu.be/ox_uDBrkp-M
"You can have this car, and everything that goes with it."
"Now you can drive the story."
There's certainly an element of information there, at least when you start getting onto companies that have an advantage in a technical area. But often, you're being sold a role: Buy this and you'll get women, wealth, etc.
When I was a girl, and still to an extent now I'm older, I wanted the things I was told I was going to like, and which it was expected that I was going to. You look at something and you see someone like you, or with some aspect of life that you want to have. A lot of the pitch towards us isn't dolls, it's friendship, or fashion, or something like that - broadly speaking, group identity.
The problem is you don't see the comparisons. If there aren't people like you being forwarded as enjoying and having fun with something, you may never try it, and even if you do the chances are that few people like you will, so you lose a lot of the social enjoyment that goes along with a thing.
Of course you're right. If girls all really hated barbies, we probably wouldn't have played with them - though I've no doubt some of us would have out of a desire to fit in. But that's not really the point. The question is how closely that approaches optimal; the most enjoyment that we could have got from a range of options, only a narrow range of which are promoted to us and will allow us to still be accepted by our friends who have also had these identities sold to them.