Countries don't want people on tourist visa to stay or work there.

So they keep a list of how much it cost per day to live there and how much it will cost you to get back to your country.

They will use it to calculate the amount you need to afford your stay and get back home. They also generally expect/want you to spend extra so they add more money on top of that.

That's how they got the minimum amount you need to have.

It's not something exclusive to the US. It's pretty common practice around the world.