No, of course its no accident. Its a pragmatic compromiose between simplicity and other prperties like witgh, stuffness, etc etc.
Exactly. Its a compromise to fabricate a general purpose element.
Elements in compression must be below Euler limits in length, or they will buckle before they actually crush:
For any specific function, you can do much better though, if you are prepared to fabricate more complex shapes than a simply rolled I beam..
Try examining the structure of an aircraft wing, to see how MUCH material can be extracted from an I beam like element without compromising its strength and stiffness.