It has always been a requirement in Tridion to have a Structure Group to enable Blueprinting. The right question to ask is "why doesn't Tridion create a structure group by default when I create a new publication?" and the answer is a complex and lengthy one...
The minor version of the reason why is that, back in the day when Tridion popped out of Arjan Van Rooijen's mind (and some others), and they decided to embrace XML and structured content as the way forward (to put this in context, we're talking before 1999), it was the intention that Tridion would be an Enterprise Content Management solution, not only WCM, in which case a Structure Group would not necessarily make sense.
Later on, when Blueprinting got added, Tridion's future was already focused on WCM more than ECM, and a Root Structure Group on the parent publication became a requirement to any blueprint tree. Adding this by default would cause compatibility issues, so there was a decision to not add it by default.
That's the short version. Should we add it into the product by default? Yes, I think so too, and it is in the roadmap with a rather low priority.