I've noticed that they aren't as rare as some think.
According to the descriptions, they are fundamentally different than gas giants. They are ice or water worlds (which of course are much smaller than giants) that have suffered a boil-off of 100% of their surface water, which has formed a thick atmosphere resembling a gas giant's. So somewhere at the core is a rocky planet.
I've noticed that they aren't as rare as some think. They are often pretty small for jovians and most I've found have only been between 20-40 Sol Masses, are similar in color to ammonia-gas giants and tend to be on the outside end of orbital groups.