Why aren't salt water batteries used to power cars instead of lithium-ion?

I know salt water batteries are less efficient than lithium-ion, but water is safer, can be found anywhere, is cheap and 100% ecological. Even if it doesn't last long, you can refill anywhere. Why can't salt water batteries be used to power electric cars?

• My guess is that energy density is too low. – my2cts Nov 10 '19 at 10:45
• Is salt water lighter than lithium? – Steeven Nov 10 '19 at 12:41

Secondly it is not true that water is lighter; Lithium is the least dense of all the metals, it has a density nearly half that of water $$0.534 g/cm^3$$.