Macro converter: maximum focusing distance with extension tube

I am using the Nikon D850 together with the Sigma 50mm / 1.4 Art and the
MEIKE MK-N-AF1-A extension tube (s).

During the first shots I discovered that the combination not only has a minimum focusing distance (which is good), but also a maximum focusing distance (which is confusing).

I have not measured it, but with the 20mil extension tube I can only focus on objects that are only 1.5 to 2.5 cm away from the lens (using both autofocus and manual focus).

I tried several aperture settings from f / 1.4 to f / 8, but there are no changes in the maximum focus distance.


Why is it a problem to focus on more distant objects?

OR:

Why does the use of an extension tube introduce a maximum focusing distance?