If we pointed any of a handful of our most powerful telescopes at it we could image it directly. But how would we know it's planet nine versus something else? Some uncatalogued star or some undiscovered asteroid? We'd need to already know where it was supposed to be. Or, we'd need to have multiple images of the same section of the sky over a period of enough days to have the image of the planet move relative to the stars and be able to reveal itself as a distant planet.

The fact is, we simply do not image enough of the sky on a regular basis to have all this imagery just lying around. We do have some pretty good all sky surveys, but they don't use telescopes big enough to see something like planet nine. That's why it takes a dedicated effort to discover planet nine. It'll take imaging huge sections of the sky multiple times with enough delay between them to be able to see separation in the position of the object. Such large telescopes are very expensive and rare so their time is very valuable and in high demand.

Distant stellar transits will happen but we're not monitoring the entire sky 24/7 to the degree needed to spot all of them. If we were we'd be able to see the planet directly just as easily.

Our view of the sky is typically just a matter of seeing through little soda straws from one day to the next, so we miss a lot.