You're operating from an incorrect assumption. Anti-tank ditches were employed throughout the war and are still used today. Here's a 1979/1980 paper from the Defense Technical Information Center (PDF) with illustrations that explains their construction and use.

According to the U.S. War Department publication "German Coastal Defenses," (Military Intelligence Service, Special Series No. 15):

"Antitank ditches have also been dug at the approaches to coastal roads and key positions, and it is not unusual to find two ditches spaced only a few feet apart. Normally, these ditches have passages to permit the flow of single-line traffic. The rear walls of these ditches are usually revetted with concrete, earth, or brick, and some of them have steel rails projecting from the top.

Around strongly defended ports will be found antitank ditches 20 to 40 feet wide and up to 3 miles in length. They are usually laid in a zigzag course. Ditches of this length are most common in The Netherlands, but they are also found to a lesser extent in Belgium and France. In the construction of such a ditch in The Hague, the Germans demolished a row of three-story houses, more than a mile long.

These ditches are usually protected by a thin belt of wire, and by gun emplacements and pillboxes sited to enfilade the zigzag course of the ditch. Where streams or other water sources are available, the ditches have a sluice arrangement to keep them partially filled with water, or to flood them in the face of an attack.

Antitank ditches installed in front of defensive positions are usually 9 to 12 feet wide and up to 8 feet deep."