@boomerang,
Cos you put your coffee on them when you're sitting down chatting.
Or your tea, or your ethanol of choice.
Or your nibblies.
I put any books on my coffee table...then they move to shelves and such.
Magazines sit there, too...and stuff.
Really, the coffee table is an organic and flexible concept.
One ought not allow concerns re what is right to put on a coffee table to interfere with the convenience of putting on it what one wishes.
I wish my coffee table had a shelf under it, though.