The whole supply of the steem doesn't exist yet. It is still being mined. Think of us like miners, we make posts and create engagement to mine coins. Hence generating more steam.
When almost all steem will be mined. It will be super difficult to mine the last few steem. And in practicality all of steem will never be mined. It will just keep switching back decimals. Like 0.00000000.....0001.

Read my blog (@amratesh) I made a post intuitively explaining ICOs. I'm planning to do one about Blockchain next. It will be more technical but I will do my best to explain it intuitively.
Cheers