The gravitational effect of a lot of spread out mass outside the Oort cloud (or indeed, a lot more mass in the Oort cloud) would have a different effect on the

The gravitational effect of a lot of spread out mass outside the Oort cloud (or indeed, a lot more mass in the Oort cloud) would have a different effect on the orbits of the planets than a single planet that we had not found yet. So for some known configuration of orbits of the known planets, there are a limited number of solutions (in terms of mass, inclination, eccentricity, etc.) that you could add to the gravitational interactions of the solar system and still have the current orbits we observe. That gives a reasonable guess about what (another planet) and where (in the sky) to look for to explain anomalies. But with distant objects that don't emit their own light, even having a good guess of a bunch of the orbital parameters doesn't mean it is easy to find, because you don't have much sunlight reflecting off of it, and the chances it will occlude something else brighter (like a background star) are needle-in-a-haystack level. Even _if_ you knew the _exact_ orbit you were looking for, there are 360 degrees of sky to search for a tiny, dark object, because you don't necessarily know where in the orbit the object currently is.