Because you might have some more pressing issues to fix than that -- it's not a good experience, but, unless you did something horrible (which newspapers often do, unfortunately), it's not that bad either. It is economy of time and attention, especially as there's always a risk that your "better" version won't be good enough anyway, and people like me will use the text extraction features of the browser anyway.

Edit: I don't mean to say that it's necessarily a bad idea to go responsive -- it's just that it's not a clear, universal win.