why does the model s use multiple batteries?
Your question is self nullifying:


  Why don't the engineers just create one giant battery?


Google: "define: battery"


  Battery
  
  /noun/
  
  a container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power.


A battery is a collection of cells.  So that is exactly what they have done. They have created a giant battery.

A battery is made of cells. A cell gives a small amount of power. Combine the cells together and you have a battery that gives a large amount of power.

It's like building a house.

Would you rather:

a) build it from bricks you can buy from the brickyard, or

b) build it out of one gigantic brick you then have to hollow out by hand to make your house?