Because that's the way Ben Hogan wrote it in Five Lessons and over time it became accepted as truth. Most don't even have a good reason to tell people to do it.
I read somewhere that Hogan only put the square back foot in because SI wanted him to address swaying in the backswing, which is a common fault of amateur golfers. I don't know whether that is true or not but you can definitely still sway with a square back foot.
My own thoughts on clubs: I used to be a big believer in getting "players" clubs once one had a semi-competent swing. I played Mizuno MP-57s for three years, all the while with a handicap typically in the high teens (and it never got under 15, except for one half month where it was 14.8).
In late 2011, I knew I'd be playing very little golf for the next two years, and I got a set of more forgiving irons. Even though I'm now better than I was when I played the MP-57s, I can't see switching. I don't need to control trajectory often enough, I don't shape shots nor do I often desire to, and GIR is just too important for me to consider getting something that may make doing that harder.