There is not enough bitcoins for everyone and that is why it's impossible for bitcoin to substitute the cash economy...
Just think about this:
A pre-defined schedule limits the total number of bitcoins so that they gradually approach a total of 21 million (ignoring those that have been lost through deleted or misplaced wallet files)
Total bitcoins: 21,000,000
Total humans: 7,360,837,156
That mean 0.002852 btc/human
but the total humans number is going up in the time. There is not enough btc for all.
that's why you can divide bitcoin into satoshies
Total bitcoins: 21,000,000
Total Satoshies: 2,100,000,000,000,000
Total humans: 7,360,837,156
That means 285293 Satoshi/human