Optics: why is there a maximum focusing distance when using an extension tube?

The lens's job is to create an image from the outside by projecting it on the surface of the film or the digital image sensor. To achieve this, the light of the objects passes through the transparent lens. The shape (figure) of the lens and the density of the lens material make these rays that form images alter their direction of travel. We can trace a trail of these emerging rays. When we do, a trail reveals that this revised path takes the form of a cone of light. This action is called refraction, that is, bending inwards.

The distance from the lens to the apex of this light cone can be measured. You will find that the light rays of distant objects reach a vertex closer to the lens than the light rays that originate from objects closer to the lens. In other words, the length of the image cone that forms the rays is a variable based on the distance of the object from the lens. The focal length of a lens is a measurement taken from lens to apex, when the lens is forming images of a very distant object (objects in the infinity symbol ∞). A measurement of the lens-apex distance made when the lens is observing nearby objects is called a "back focus."

The key to answering your question is the fact that a lens has a limited ability to refract (bend inward). When we focus the camera, we are adjusting the distance lens to the sensitized surface in an attempt to make the vertex of the image cone that forms rays simply kiss its surface. If the distance is correct, the object will be focused. If the focus adjustment makes this vertex too short or too long, the resulting image will be blurred.
Again, as we focus, we are changing the distance from the lens to the sensor. The focusing mechanism of most cameras limits the amount of lens-sensor movement. This is because most camera lenses are optimized to work distant subjects. The design of a macro lens is optimized to work closely and slightly compromised when assigned to work on distant subjects.

Standard camera lenses generally keep forward movement at a distance of approximately ½ meter (20 inches). If our desire is to obtain super close images with a standard lens, we must install tubes / rings or bellows or resort to the use of complementary foreground lenses.

We commonly use extension tubes when working nearby. The tube simply increases the distance from the lens to the sensor. In other words, they lengthen the lens at the apex distance, and this accommodates the rear focus distance now stretched. With a tube in place, we get checked lenses for the subject's distance values. Because the lens has a limited ability to refract, the focus range now reviewed is limited to foreground work; We can only work a narrow space of close distances between objects.