Why is there no external male reproductive organ in a frog?

Frogs reproduce by a special kind of embrace called "amplexus" which is an external fertilization that does not occur inside the female's body.

During mating (amplexus), the male frog grasps the female frog's trunk with his forelimbs.

The female frog discharges eggs into the water and then the male frog sheds the sperms over those eggs.