I'll just copy part of my other comment:

As another comment pointed out this is in part due to racial majority being defaulted into the non-specific term "american". As in: most americans are white so "americans" is thought of as refering to the biggest group of americans.

And in part due to historical subjugation of those other "americans". As in: less than 80 years ago the term "americans" was used almost exclusively to refer to white americans due to systemic racism.

It is still particularly prevalent in media headlines.

I don't know why you think I would make this up, lol.