...because there is nothing outside of it.
If you add up everything with everything else, all of whatever exists in all of existence, it's 1. The sum of everything.
When we want to explain that something is smaller, we uses pieces, or decimals, to describe it. Add them all up, and it's always 1.
So the bigger a number seems, the smaller a thing gets.
I think about this when I am in a million pieces, as I am now. They will all fit back together, and I will be 1.