Sure the Great Lakes have the St. Lawrence River to drain it and many lakes are the sources of rivers, however some lakes or inland seas are where the rivers flow to and only evaporate or fill the lake.

Think of Lake Baikal, Aral Sea, the Caspian Sea, even the Dead Sea. Not all waters flow to the sea.

I'm not countering the arguement that FT like most terrain generators have all water at sea level only is the real problem - just making a point.