Who said “ flat freq response “ is the best?

Human studies.  
 
If your gear measures perfectly flat on-axis in an anechoic chamber, and it has ideal off-axis (steadily and evenly sloping the further you go), then it will sound good in a “typical room”.   
  
Now, the room does play a large role, if you are in an untreated room that has tile floors and large windows, you may want a speaker that has a recessed treble.