I'm surprised nobody mentioned this yet, or maybe I just missed it. Here is an example of why we don't use air instead of CO2:

http://www.genuineinnovations.com/bi...px?prodid=1011

The product listed above will use either CO2 cartridges, or "Big Air" cartridges (which are filled with.... AIR). Notice how BIG the Big Air cartridges are.

So to answer the actual question: Because air cartridges would have to be a lot bigger than CO2 cartridges.

And to answer the question that wasn't asked: There ARE alternatives that use air, but they're not very popular because of the size.