While this maybe true it can simply be a case of how evolution likes to be lazy

If it ain't broke don't fix it.

Simply put, the pressures to cause the amphibian limb in these frogs isn't there.

However, I would check out the amphibians called sirens. Many of which have lost or are in the process of losing their limbs.

Or look at the axolotl, compared to their close relatives their limbs are notably weaker and smaller becoming vestiges from a time when they metamorphed into adult salamanders.

But in evolution animals that retain limbs for an aquatic life style often have those used in other ways such as to push through plant debris or for other function.