Most people have never set their preferences and don't know that they can view full size as a default and aren't brilliant enough to click the image for a full size view;

They are typically brilliant enough, but may not be aware or choose not to becuase it is an extra bit of effort.

Which doesn't explain why they chose to implement it that way, but, eh, what the hell.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)