Question

why cant we see objects which are behind us

Answer 1

Because we see objects due to reflection.And objects behind us don't get reflected from our eyes.

Answer 2

heyy mate here is ur answer

==> Our eyes can detect an object when the light emitted by the object strikes on our retina. ... If we can mange to make the light from the object behind us to reach our eyes (like using a mirror), then we can see the object.

I hope it will helps u.