They are far too dim. Remember that light falls off with 1/r^2 distance from the Sun, while at the same time the intensity of light received at Earth falls off with 1/r^2 distance from the Earth. For objects in the outer solar system many tens of AU away from both the Earth and the Sun the result is that the distance from the Sun and the distance to the observer (which is usually near the Earth) are similar, since at such scales the Earth and the Sun are very close together. This means that brightness falls off roughly with a relationship of 1/r^4 distance from the Sun/Earth. Meaning that an object 100 AU away is not just 10,000x dimmer than 1 AU away but 100 million times dimmer.

We can just barely see giant balls of rock and ice that are hundreds of km across in the outer solar system, a tiny bit of metal just a few meters across at most is basically invisible to our optical and infrared telescopes.