Evolution by natural selection can explain why frogs jump instead of walk.
FOOD. Frogs eat bugs and leaping at them quickly yields greater likelihood of scoring a meal than the alternative suggested in the question: walking
PREDATOR/PREY SURVIVAL. Frogs must avoid being eaten by birds, fish, turtles and other creatures. The quick jump reflex gives them a greater chance of escaping a predator than the suggested alternative in the question: walking.
Living with these environmental factors gave greater probability of survival and reproduction. Genes equipped with the advantage (jumping ability) prevailed.