Because what makes it useful , like media queries, can't be put in style attributes. Frankly this whole discussion is just as silly as claiming flex has nothing over table layout.
Then again one should aim to not even need media queries and make things responsive intrinsically, breakpoint-free, continuously. A design where things jump at hard coded viewport widths is not actually a great design.
>inb4 my 1:1 naive mapping of DB to screen works for me
>inb4 the web should be reserved for text documents and hyperlinks and user styles rule and running anything close to applications on the web was a mistake