Hi mustard-tree!  In general terms, there are two reasons for this; one is theatrical, and the other humanitarian.  But from what I've seen from the US media, it's the opposite situation that more often applies, and not the one you mentioned.

Consider television, the movies, and the media:  It's all mountains of hype!  People in law enforcement do not go around making grandiose melodramatic gestures all day long - it's a job.  But just doing a "job" is boring, so the media spices everything up.  They go "Hollywood!"   They even have weird "mood music" playing in the background to amp up the suspense, to make everything even more dramatic.  Anyway, writers, biographers, news reporters, etc., have to find a hook!  And having a man on death row, or even better a woman, suffer this way is a wonderful hook, great for the ratings.   This kind of television helps to explain why the entire nation needs shrinks so badly.

If you want something closer to reality, please watch a movie made some time ago, titled "Show Time."  It had Eddie Murphy and Robert DiNero - not a bad cast, really!  Bill Shatner was in there too, and he got some laughs.  The movie shows the differences between Hollywood and reality, or at least some of them.

And then, there is the humanitarian angle.  Nobody really wants someone to unduly suffer.  But some time ago, the ACLU published something about death row inmates being slighted and even denied basic medical services, such as dental care, long term medical treatment, and so on.  It has been many, many years since I read this so, unfortunately, I don't remember any details.

I hope this helps.  I'm not sure of the answers either.  But this is my guess.