The reason it appears as a visible dot is because camera lenses cannot perfectly focus on a light source. You know how a point light source looks like a circle in a photo when it’s out of focus?
Well, even when a point light source is in focus, there is still a small amount of spreading or blurring. It’s just that the circle is usually so small that we don’t notice it in the image. Here, the light source is incredibly small, so the spread of light becomes noticeable even when in focus. Otherwise, only a single pixel on the sensor would be illuminated.
PhD candidate David Nadlinger captured an image of an atom lit by a laser and held aloft by two electrodes using a long exposure setting on his camera. The two electrodes on either side of the atom are spaced 2 mm apart to give you an idea of scale. The atom absorbed and reemitted the light particles that the laser projected, which is why you can see it. (University of Oxford/EPSRC/David Nadlinger)