Weird maybe but most people wont be downloading their forum software from a mobile phone and for the most part once you are a licensed member here that is pretty much the only time you ever go to the client area (to get your software or file a ticket).
Could it be responsive, yea sure and it would look great.
With all things considered is it going to effect me, no not at all...I never shrink up my browser when viewing the customer section and I don't upload and install xf from my phone so..yeah, score.
The page could be adjusted but there is no reason for "responsive". Responsive implies its using media queries to change css based on width. That page is 960 pixels minimum and if you pull that the real minimum width is around 655 pixels. A minor adjustment so that the 4 info grid stays parked left and the page goes 2 column it would still look pretty damn good on mobile without any "responsive" code. Less than 655 pixels would require responsive because of the elements in the header and footer. But as has been said many times its not really something one uses on mobile.