21 Million isn't just shared by 8 billion people, it is also shared by past people and all future people.

There are 8.1 billion people currently on earth. There are already some people that bought early and have already died (Hal Finney for example), so they no longer count in that 8.1 billion. People have already died with their keys.

All future people born will be sharing this 21 million pie as well.

Humans have existed for 200,000 years. 109 billion people have died in that 200,000 years.

Earth will be uninhabitable in 1 billion years. So 200,000 years from now, perhaps we can assume another 100 billion people.

All those 100 billion people have to split the 21 million pie.

Even if you have 0.0001 btc, that's still a lot