Why is there a need to hire ‘A’ grade employees for emerging start ups?

The “Art of The Start” by Guy Kawasaki mentions the following famous saying of Steve Jobs , on the need to hire ‘A’ grade employees , for any emerging start up:

‘A’ grade employees would hire ‘A’ grade employees , ‘B’ grade employees would hire ‘C’ grade employees , ‘C’ grade employees would hire ‘D’ grade employees and soon the organization will be filled with bozos of no use.