Why do people buy 5k TV sets and then go home and plug it into an 1080p cable box or stream at most 4K movies?

It is the "L&G Effect."  Latest and Greatest.  When a new product comes out that is touted as 'better', 'faster', 'more powerful.' there are some people that will drool all over it just to get it, even though the current cable streaming or other data they have at home is less.  it's all about wanting to own what's L&G. 

It's called "future-proofing" and it's done all the time.  Sometimes it's worthwhile and sometimes it's not.  In a technology "revolution" most of us buy one piece at a time.  Sometimes the "new tech" is abandoned.  (like 3D TV)   

chronic retardation

There's something to be said for being future ready. 8K is being slammed by tech people as being totally worthless for regular home owners though.

They'll have to think up a new selling point soon.

You mean 8k?   They do that because 8k content is rare.  You won't have much of a choice but to watch 4k and 1080p streams for years to come if you purchase an 8k TV, which is why they are a complete waste of money compared to just buying a good quality 4k TV. 

some porn is in 56 and 6k

They don't know any better.