Because the focal point of this is the stream power, not the döners. So, having come up with steam power 2000 years before the Turks is more relevant than migration patterns and cuisine.
There have been quite a lot of Greek guest workers in Germany after WW2 and many stayed here just like the turks.
There is only a German Wikipedia article about this deal, but before the turks came they first had guest workers from Italy and grece in West Germany. https://de.m.wikipedia.org/wiki/Anwerbeabkommen_zwischen_der_Bundesrepublik_Deutschland_und_Griechenland
