The biggest reason that the Germans built so few of their tanks was a function of time.

The first British tanks did not debut until mid 1916, and their performance was not exactly awe inspiring. They were plagued by performance and reliability problems. It was only after several iterations that the British were able to claim that they were making a real impact on the battlefield. It had taken over a year for the British to get their tanks from the initial drawing boards to the battlefield, and even longer for them to appear in any number with the first of the Mark IVs (the most populous British tank of the war) only appearing in April 1917. Over that time they had made countless changes to the tanks themselves as well as gained a considerable amount of experience in their use.

It was not until November 1916 that the Germans decided that tanks were something that they should invest the large number of resources required to produce. They had some captured British tanks to help inform their design decisions, but they still had to go through quite a lot of the design process. What would come out of the German design process had a very different set of performance parameters and design decisions than either the French or British. The Daimler company did the design and production, but there was not a lot of time for them to try and work through the various design and production problems. It did not help that their resulting tank was of a design that was probably less useful, in my opinion, than what the Entente were doing. Other than the "Mark" tanks the Entente, and especially for the French, were going for faster and smaller tanks. On the other side the German A7V was really big and really heavy. It had a lot of power, and was very well protected, but it was still very clumsy on the battlefields of WW1. Cooper would describe its performance as "Although it had a higher power to weight ratio than the British tanks and sprung tracks for greater speed, it proved a failure. Its high centre of gravity, fore and aft overhangs, and short tracks made the [...] A7V a poor cross-country performer and incapable of crossing trenches." (excluded a few German words from that quote) A lot of the design of the tank reminds me of the series of events that resulted in the Porsche Elefant in WW2, too big, too immobile.

However, even if the A7V would have been a T-34 it probably would not have made a difference. They were not available until October 1917, and there were only 10 available for the Spring 1918 offensives. After these offensives there were only a few months left in the war.

Opinions and Pieces I Don't Have Sourced:

1. I think the German economic situation by 1918 was a contributing factor for how many tanks could be made. A lot of steel and other materials went into each tank. However, I don't have a source for this and would love to see a German focused look at the tank if anybody has one. The two sources below are HEAVILY British focused.
2. I also believe that one of the problems for the German tanks was that the German High Command did not necessarily believe them to be necessary. I haven't done any research on this yet, my research currently stops at the end of 1916.

Sources:

Devil's Chariots by John Glanfield

Tank Battles of World War 1 by Bryan Cooper