The short answer is, we created it. The half-life of Uranium 235 is 700 million years, so there's plenty of it around. When you bring a bunch of U235 together

The short answer is, we created it. The half-life of Uranium 235 is 700 million years, so there's plenty of it around. When you bring a bunch of U235 together and cause it to undergo fission, it creates a bunch of the shorter-lived stuff as a decay product. Especially when a reactor melts down; you get a lot more of the stuff you didn't want, in a much more uncontrolled manner.