CSS is a syntax for specifying the appearance of text and other web content, sizes and lengths are defined in pixels, however pixels have different sizes in different devices, this is the most notable in mobile devices.
If the developer wishes to specify lengths in distance units like centimeters or inches, the CSS specification and its implementations allow the developer to specify their wish but fail to keep their promises. If a box is defined as 1 inch wide, the browser will convert the inch unit to pixels using a fixed ratio and will produce a box 72pixels wide, which will be of different lengths in mobile and desktop displays, none of which will be 1 inch wide.
The question is two fold:
Why can't developers specify lengths using real world SI units?
Why does css appear to support SI lengths but fail to do so? Admitting that it cannot do so would be better than lying. It appears there is a history of a failed feature here.