The reason frog fossils are hard to come by is because of the
softness of their bones.
When the earth, soil and rocks move underground, their bones are easily destroyed. Which means fossils cannot form as easily.
But saying that, one of the most famous fossils ever recovered is one of a frog, as you can see its facial features, which is rare for a fossil in general, let alone a frog fossil. More about that on the national Geographic website (click here).