Tradition/upbringing. Early Americans came from central & northern Europe where those traits are more common, so it became the idolized “norm” and a way to distinguish from the “new” Americans from eastern/southern Europe & nonwhite countries where dark hair and eyes are more common. The US is more mixed now but that trait is still associated with the “all American” type and “success” because of how long it’s been around. So people grow up seeing that be called beautiful and internalized it.
#5 Not all women are beautiful, just like not all men are beautiful. But beauty is not tame, controllable, or predictable. It does not follow a formula. Hitler thought it did, and I think if he was actually right more people would have agreed with him. Fortunately he was wrong. Beauty can happen in many shapes and sizes. It happens more often amongst those who live in peace and live well, which are disproportionately places where wealth has accumulated (thus the mistake Hitler made).
Also that’s not me saying EVERYONE thinks this. Plenty have other types, especially those who’ve grown up around a wider variety of ethnicities. I notice it more in more homogeneous mostly-white (especially upper middle class/old money) spaces. People who have that idea just tend to be a bit louder the same way people who prefer someone more slender are because it’s like the pop culture of preferences. Indie bands can get big or be popular but most people will still know Taylor Swift.