galaxy - Why can't we see distant galaxies with the naked eye?

Not at all a dumb question, but actually you can see distant galaxies with the naked eye. From the northern hemisphere, the Andromeda Galaxy, our biggest neighboring galaxy, is visible if you know where to look, and is at a reasonably dark place. From the southern hemisphere, the two smaller, but nearer, irregular galaxies called the Small and Large Magellanic Clouds are visible.

The reason that more distant galaxies are not visible, is due to the inverse-square law: As the light particles (photons) recede from the galaxy (or any other light source), they are distributed over an ever-increasing surface. That means that a detector (e.g. your eye) of a given area will catch less photons, the farther it is placed from the galaxy. The law says that if in a time interval Δt on average it detects, say, 8 photons at a distance D, then in the same time interval, at a distance 2D it will detect 8/2² = 2 photons. At a distance of 4D, it will detect 8/4² = 0.5 photons. Or, equivalently, it will need twice the time to detect a single photon.

The bottom line is that in principle you can see the very distant galaxies, but the photons are so few and arrive so rarely, that your eye is not a good enough detector. The benefit of a telescope is that 1) it has a larger area than your eye, and 2) you can put a camera at its focal point instead of your eye and take a picture with a large exposure time, i.e. increasing the Δt.