Why must website elements resize magically when windows are enlarged?<p>People read best when they don't move their head left and right. This means 80 characters in a line. Why must CSS frameworks have support for being "responsive"?<p>Why is responsive good?
You've misunderstood what "responsive" means.<p>Text growing to fill the width of the page is the <i>default mode</i> of the web. The ability to control the width of text came much later. What has only recently become practical - and what people mean by "responsive" or "fluid" layout - is the ability to intelligently reformat the page to suit the available container, by for instance using a layout that can grow from a single column to five or six columns depending on the container width, or by removing or simplifying certain navigation elements in very small containers.<p>Designing around a fixed layout is a very bad idea. Viewers with smaller than anticipated containers will have to scroll horizontally and users of wider than anticipated containers will be frustrated by large areas of whitespace. Your layout is liable to break completely if a user is substantially altering your page layout, for reasons of accessibility or convenience.<p>Responsiveness is about designing sites that are maximally readable and usable on all devices; When done well, sites will effortlessly adapt to new devices and new form-factors.
Having two separate websites is bad for user experience no matter how you look at it. I approach it to having a flash website and an html one. Same concept. Plus mobile detection is not future proof.
The real reason? Because making sure that your web site has a browser AND mobile AND app version is too expensive. So most sites settle for a crappy responsive version that pisses everyone off to some degree.