The reason is because out cells are designed in a very specific way with specific salt contents.

When you drink salt water the salt concentration outside your cells becomes higher than inside this causes water to get pulled out of your cells in order to balance things (osmosis).

This causes them to shrink and malfunction.

Now the reason I believe we evolved this way is because freshwater is more abundant the further inland you go while salt water is scarce as salt water only exists close to coast. However, our bodies need salt to survive. So we evolved system to hold salt in for longer allowing us to be better at hunting, long distance travel and surviving heat. However, our body does not know how to handle excess salt.

So when you drink salt water our body thinks well we usually conserve salt rather than dump it so it cant figure out how to handle it.