Why does the European Plain give Europe a low average elevation?

The European Plain is one of the most extensive uninterrupted expanses of flat land on Earth, stretching from the Pyrenees in the west to the Ural Mountains in the east.1

Because this massive lowland covers such a significant portion of the continent's total territory, it results in Europe having the lowest average elevation of any continent. While the terrain is occasionally broken by hills or glacial features, the vast majority of this region remains at a low elevation; for instance, the East European Plain is consistently below 1,500 feet (450 metres) in elevation throughout its entire expanse. This dominance of low-lying, relatively flat land defines the continent's overall relief, distinguishing it from regions dominated by high-altitude mountain ranges and plateaus.123