Shape is largely restricted by the limitations of manufacturing and the laws of physics, but there is also a fashion aspect as well. I mean very few modern aircraft look like the Wright Flier with its canard foreplanes and rear mounted straight wing and engines belt driving pusher propellers. Once a stable layout was found it was stuck to until something required a major change. The layout of a spitfire is not actually that much different from a Foker Triplane of WWI. The difference is in the emphasis on lift vs speed and a large increase in engine performance. With the introduction of the jet engine the common design gradually moved the engine from the front to the rear and swept the main wing but everything else pretty much stayed where it was for most aircraft.not just for that - figure why comercial jets look the same - Boeing - Airbus
the f-22 and f-35 and even f-23 look the same because of the computer generated design - when u have a input data of a radio wave that u want to avoid - the answer will be the same - a shape like f-22,23,35
BTW we should get a few things straight however the R-73 and R-73M2 look nothing at all like an IRIS-T. The picture shown of the New Russian SRAAM is of an R-74 which is currently still being developed... as is the Medium range R-77M and the long range missile... either the R-37M or the KS-172.
If it makes you feel better the IRIS-T looks a lot like a slim STANDARD II American naval surface to air missile with regard to its control surfaces.