>>11707342There are two factors that make space objects hard to see: size and brightness. Telescopes help overcome the first with magnification (stronger lenses), and the second with aperture (larger light collecting area) and exposure time (let more light hit the sensors). Many galaxies are big enough that you don't need much magnification at all - and bright enough that a 6 inch telescope and a few hours' exposure will give you a nice image.
Size isn't much of a problem on its own if you're simply trying to detect something without resolving any fine features: stars are a great example of this. You can try to magnify a star by a huge amount, and it'll still look like a point. But they give off so much light from that tiny point that just increasing aperture and exposure will let you see very distant stars. So the biggest barrier to detection is brightness.
Distant comets have some of the worst of both size and brightness. Sunlight has to travel hundreds of AU out, diffusely scatter off a rock maybe a few miles across, and travel hundreds of AU back to us - decreasing by the inverse square law the whole time.
So even though comets are much closer to galaxies, they are many orders of magnitude dimmer to us. This would require unfeasible apertures and exposures to detect. And their miniscule apparent size offers essentially no hope of occluding distant stars for detection, or resolving surface details.
