Mostly this. I visit a website to view its content, not its header. I don't need to waste 10% of the screen on something I will almost never use. On Desktop, I can press "Home" to get to the header in the rare instances I want to. On Mobile, screen space is even more valuable, so it's even more annoying to have to waste screen space on the stupid header (and no, web designers, the style that pops up and covers up the content you are trying to read when you scroll up one pixel is not a solution).
I'm here to read. Stop interfering with that.