Why did calais landing seem logistically sound to the germans?

The Germans thought Calais was the likely landing site for an Allied invasion because the Allies made them think that's what they were planning.

The Allies employed various strategies, code named "Operation Fortitude," which were intended to deceive the Germans in advance of the planned Normandy invasion.  They built up phony infrastructure for possible invasions, on at Calais just across the English Channel, another as a northern incursion via Norway.  They also leaked phony intelligence reports that would find their way into German hands.  So Calais seemed logistically sound not only because it was directly across the Channel from Allied positions in Britain, but because the Allies were wanting the Germans to think that was a possible plan they were making.  The deception was meant to give the actual invasion at Normandy a better chance for success.