After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels. The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean.
In the absence of fresh water, it had also adapted to drinking salt water with a higher salt content than sea water. Domestic Bactrian camels cannot drink salt water with this degree of salt. Research to date does not show conclusively how the wild camel is absorbing and secreting the salt water.