Because they can't see it

Planet 9 may not be a planet at all, but rather a 'primordial black hole,' shocking study suggests

The possible existence of Planet Nine and its impact on distant objects in the solar system has fascinated researchers for some time. But a

They know where the gravity well is. They cannot find anything sitting out there waving back. Thus, it must be a black hole. It has to be a bit of neutron star. It has to be a gravitational anomaly. It might just be a planet that doesn't reflect much light. until it commits the appropriate behavior for us to see it, it remains invisible.