Unenriched uranium will fission eventually, but you're talking trillions of years. In a fission reactor, the U238 atoms are placed close enough to a minority of U235 atoms and to each othekr that neutrons from one fission will trigger the next at a rate just short of running away exponentially.

There are some ore bodies that are concentrated enough to fission faster, but they are rare (and not the uranium that is typically mined for fuel).