Because that is exactly what it is -- a "dead" sea, no disrespect intended. Nestled in the Holy Land, the Dead Sea, sitting below sea level (deep), actually has so much salt content resident in the water that literally no life form can live in it. Thus the name... the Dead Sea.
P.S. You cannot actually float on top of the water without trying, because of the mass salt content!
It was a pleasure and an honor to visit the Dead Sea, Jordan, for the World Economic Forum session in May, 2005. Go and experience it for yourself... Jordan is worth the 21 hours of travel to get there!
Onward, with HOPE