There can be two possible reasons for this:
1. Like Truman Sharp said, frogs are placed lower in the food chain, and a lot of them need to hopping around to sustain the life above them.
2. This seems more likely though:
Amphibians like frogs reproduce by external fertilization. That is, the female and male gametes fuse outside the frog's body. This usually happens in water, thus there is a possibility that the male gametes never make it to the eggs, unless there are lots of them. This could be one of the reasons that frogs release so many gametes.