As I understand it, we don't have enough information about them to say. It could be that they're extremely rare and only occur naturally in places with lots of chaotic gravitational forces, or they could be relatively common but imperceptively tiny and lasting for only very brief moments. Imagine if on the surface of stars, the gravitational fields occasionally crash together in such a way to create a wormhole that is big enough and lasts long enough to deposit a single hydrogen molecule at a random point in the galaxy. That would be considered relatively common when compared to the possibility that it would only happen in areas where two black holes are colliding or maybe where two stars are colliding. Our technology isn't good enough to detect that kind of stuff