As was noted, they have many, many excuses for that. I'm often more interested in how they would answer why they don't feel like their actual gender. I've never seen that answered without involving vagueness around feelings and a large number of gender stereotypes. Oh, and usually anger.
Growing up, I never actually thought that much about my gender. I was a Tomboy because I wasn't into girly stuff but was more into boy stuff. A female was a female, a male was a male, some females like more male things, some males liked more female things but they were still as they were.
My brother went through a stage of dressing in dresses at about the age of 5, he quickly grew out of it. I was always in jeans and a t-shirt, hated dresses and as soon as I was able to not be forced into them, I've not worn one.
There was always a natural curiosity, 'I wish I was a boy' etc etc etc but it was the same as, 'I want to be an astronaut/policeman/fireman/vet, own a pet lion to eat my brother'. Whimsical wants, stages and the imaginations of a child who knew no better than, 'I want, I need, I wish'. At no point did my parents buy me a pet lion because its wrong to a) own a pet lion and b) have a pet lion just to make it eat people, when I said I wanted to be this or that and they knew it wasn't achievable, they steered me away from it. I wasn't allowed pizza every day just because I wanted it, I wasn't allowed to join the circus, I wasn't allowed to live in the stable with the ducks and pony we had because as it turns out, I wasn't a fucking pony.
I don't think anyone can answer the gender question because you just are. I am female, I was born female and that's what makes me female, I wasn't brought up to question the validity of my gender, my sex. Just because I am more masculine than feminine does not mean that I am not female, it just makes me a more masculine, heterosexual female and that's it, the end. I've known effeminate men, from the same era as me, they are straight, effeminate men, they never questioned their gender or sex because it was never an issue, they were just effeminate boys that grew into the same guys.
I'm not sure the question is, 'why can't they answer', I think the question should be, 'why are people questioning their gender based on having a few traits of the opposite gender and why from such a young age?'.
