You "can" swap out parts and repair iMacs, it is just not something one does casually. You need tools, instructions, skills and will-power.

The thing is most people, even pros, don't bother with that stuff even when they have Dell boxes. Apple knows that and, I think, trades off ease-of-repair for sleekness and mostly wins out (until recently).

AFAIK, there is no "screen mode" that enables one to use these things as monitors.