How can a large passenger aircraft's owner remain unknown?
Every country handles aircraft registration differently (but generally similar). Im sure Boeing could tell you who they originally delivered the airframe to but if it was sold from there it could legally go anywhere and would only be subject to local registration legislation. 

Here in the US you can look up aircraft owners by N number but not all countries may keep a record like that and as far as I know there is no global aircraft registry. 


  it is still impossible to find actual aircraft's owner in a matter of minutes


Knowing who the owner is and finding the owner are two very different things.

According to this article  


  Several aviation databases list the Boeings -- identified by their
  call signs TF-ARN, TF-ARH, TF-ARM -- as belonging to leasing firm Air
  Atlanta Icelandic, but that company says it sold them in 2008.
  
  Since then, the aircraft appear to have changed hands several times.
  
  Malaysia Airports says it's entitled to sell the Boeings under the
  country's civil aviation regulations if no owner comes forward.


So it seems they do know who owned them at some point (most likely the last people to put readable registration numbers on them).