Question

Why can't we breath under water?

Answer 1

Our lungs are not designed breathe oxygen from water to be able to breathe under water.

Answer 2

We can certainly draw water into our lungs, even though
our brain screams "Don't do that !".

But our lungs can only separate oxygen out of air, not out of
water.  So if there's only water in our lungs, we pass out after
a short time because we're not getting any oxygen.