As said earlier, just because they are major websites doesn’t mean they are the best practices.
Well it still doesn’t make sense to define a font size in inches on a screen, and I think that’s why they stuck with px.
It doesn’t make sense to define fonts in pixels because then the size it will display at will depend on the screen resolution and may be way too small to see if it ends up where there are say 200 pixels in an inch and you define your font as say 12 pixels. That means your font ends up as just over 4 point which is so small that almost no one will be able to read it at that resolution. Since some browsers can’t resize content defined in pixels that will leave those people unable to read your page.
You shouldn’t define the font for the screen in inches of points either. By defining it in em you allow your visitors to decide how big they want an em to be and all of your content then adjusts itself to that size while maintaining the same basic layout. Your visitors can therefore resize the page so as to make the text big enough to read or so as to fit more of the page on the screen at once without breaking your page layout (which is what happens when you resize it when it is defined in pixels (in those browsers that allow it).
Keep pixels for borders only where you want to define borders as thin as the browser can display - one pixel. Do not use them for anything else - define the rest of your content for the screen in em and % so that they will maintain their relative sizes when your visitor decides to make the font bigger.