Why was the main trade route passing through the lands of the Eastern Slavs called from the Varangians to the Greeks?

Yes, indeed, the trade route from the Varangians to the Greeks passed through the territory of the Eastern Slavs, but it was not paved by them. At the same time, of course, the Slavs, like many other peoples of the European part of the continent, used this strategic path. Varangians are Normans, they are also called Vikings. The Varangians are inhabitants of Northern Europe, excellent shipbuilders, robbers, but also merchants at the same time. Along a number of rivers, crossing the continent, they sailed to the Greeks, that is, the Byzantines. The Byzantines called themselves Greeks.