They DO horizontally scale, just not very cheaply. Reliability above all else is their advantage. There is an entire internet between you and a "5 9s" cloud provider that can screw you.

side note; I HATE the trend in the level of abstraction away from hardware. Seems about as short sighted as GP doctors abstracting away human anatomy. They mostly treat illnesses and proscribe medication; why should they need to know the names of bones in a human body? There are orthopedic specialists for that!