That could be part of the answer but because the oceans are so vast there's a much bigger chemical process going on than in a lake. And there are also some lakes that are extremely salty, for example the Great Salt Lake. For various reasons because of the nature of the minerals that feed into the lake, the amount of evaporation that goes on in the lake and because the Great Salt Lake is in the desert, water flows in and then evaporates away and leaves the residue behind.

Whereas in lakes like the Great Lakes there is more of a turnover in fresh water: the water flows in and goes back out again so there is a flushing. The water eventually makes its way to the Saint Lawrence River and to the ocean. So the Great Lakes get flushed through and the salt doesn't accumulate. In the Great Salt Lake there's no outflow (i.e., the water flows in from a river and then evaporates into the atmosphere and leaves the salt behind). This is the same reason why the Dead Sea is very salty.