I think the problem isn’t even that they have to keep the characters pretty. The problem is that they define female prettiness as an absence of features.
It’s like the bad-anime face.
The total lack of identifiable human features forces you to project idealized features onto their void of a face. There are enough face-like qualities for the mind to recognize that a face is supposed to go there – but with no specific information, your brain picks all the features it likes the best. Whereas if they tried to make a female character actually modeled off of a real female face, like Angelina Jolie, they have to deal with the fact that not everyone finds that specific face attractive.
I think that’s the core problem with objectification. It’s not just that women are sexualized – when you watch cartoon films like Hercules or anything by Don Bluth, the men are designed to be sexualized and pretty too. It’s just done in a radically disparate way, in which women are sexualized not according to their individual characteristics, but by having their individuality covered up. It’s essentially the same mentality as a burqa – an attempt to define women as only being one particular set of things through a campaign to hide their actual attributes.