Question

why do amphibians do not possess exoskelton

Answer 1

Because amphibians has endoskeleton....and moreover they don't need exoskeleton..for anything...the basic purpose of any skeleton let it be exo or endo is to provide support and protection to the body....
Moreover they have moist skin that also make them slippery and not easy to catch by predators.......so they don't need any exoskeleton for any purpose
I hope it helped....

Answer 2

an exoskeleton is a hard outer covering made of hard, semi transparent substance called chitin.
chitin forms the exoskeleton in arthropods including insects, such as spiders and in some fungi and bacteria