Same with Uranium!

The reason is simple but bit insidious - when there is enough "known reserves" of something, it is not economical to look for more. Once the amount of known reserves shrinks, it beings to be worth it and people start looking again. So it might always look like we have between two to five decades of oil or uranium or lithium or whatever.

Realistically, we will end humanity by burning oil a lot sooner than we will run out of oil.