system
Given the following post, respond as if you're commenting on Reddit:
human
If seawater is much more abundant than fresh water.. why did we evolved to not being able to drink it?
gpt
We actually sort of did. 

Our lineage (the chordates) first appears at the basal Cambrian about 540 MY ago. Back then, our ancestors were aquatic and pretty much  confined to salt water environments. It isn't before the Devonian, roughly 170 MY later, that one of the lineages within the chordates (the tetrapods) began transition towards land. 

Now moving inland requires a the acquisition of a certain number of specialised caracters, one of which is the ability to use fresh water. So the ability to use seawater was pretty much discarded in exchange for the necessary capacity to survive on fresh water at that point. 

This adaptation is by no way trivial, BTW, as anybody which has transferred oceanic lifeforms to freswater aquaria can attest. Unless you have some adaptive mechanism, osmosis will just bloat you up till you explode.

Also worth noting that some members our tetrapod lineage have since moved back to the ocean (Cetaceans and pinnipeds) and have had to reacquire the ability to drink seawater.