Why aren't the passenger seats designed as jump seats (foldable) in commercial airlines?
You're in luck. Airbus has filed a patent for that. (Not a joke.)


  Airbus said the design was a response to recent demands from low-cost airlines to cram more seating into existing plane interiors – often to the detriment of passenger comfort.


Just when I thought the below can't get any worse.

(Original, sardine edit by @TomMcW)



There is also this concept:




  Although the patent has been filed, whether or not the seats will become popular depends on whether or not airlines want to pick up the design.