Why are there so many aircraft designs which look generally profoundly similar?
It’s an interesting question.  Aviation historian Bill Sweetman wrote about just that in an Air &amp; Space article called How the 747 Got Its Hump.  The answer to the question is quite long and specific to the aircraft type and use but the short and sweet answer is that airplanes are a great compromise between a number of factors, including mission profile, aerodynamics, structures, propulsion, avionics, manufacturability, realities and practicalities of product use, and on a few occasions customer insistence and industrial espionage.  See illustration below.



It just works out that, when all the factors above are taken into account, the designs you describe tend to be the best all around solutions for particular aircraft sizes, costs, payloads, mission profiles, safety, etc.  Like any other artifact made by humans, aircraft are but single answers to open ended design problems; a compromise between all of these competing requirements into a tangible product.  No one answer is 'correct' per se, but these kinds of aircraft designs seem to be the most optimal and every single design team when confronted with the requirements and design constraints create much the same thing.