Why do people who don't play chess have a chess set in their living rooms?

Same reason some people who don't play piano have a baby grand.

for a decoration , seems everyone notices a chess set .

It is a decor, and they are using it as a novelty item. It is the same as keywind wall clocks/grandfather clocks, they obviously don't keep time and are early 20th century technology but people has them anyway to make their house look pretty. 

Because chess is considered to be  a game complicated enough to be only of interest to those with an intellectual  approach to entertainment.  Of course, this is a load of bollocks.  However, those who consider that it produces this impression are quite willing  to try to give this impression  to those who should know better.

Same why some people who barely read books are having massive bookcases, with several volumes of classic writers and so on. Decoration.

for a decoration , seems everyone notices a chess set .

So that people who do play can play when they come over.. Same as a pool table...

Probably, got tired of losing so they stopped.

I played some people that are really good. The fun disappears when you see your pieces.

Sometimes, the games suddenly ends and you find your opponent's got you on check mate.

a e s t h e t i c

Maybe only one of them actually plays. I have a chess set in my house that gathers dust until friends and family come over and we dust it off. My wife doesn't play.