If you're questioning why no one builds it as a second item outside of boots, I can say that 99% of the time, I don't, even on the junglers you mentioned. The problem with getting that item immediately afterwards is that, very likely, an enemy champion or two is spiking mid game and resistances are really important. For example, let's say the enemy has Rumble and Corki. Then, getting your huge MR item or at least a Specter's Cowl does so much more than RG or Catalyst for you.
It's also worth mentioning that Catalyst as an item doesn't give the greatest brawling stats mid game and as a jungler who needs to maximize the gold you get early to mid game, it is a pretty weak transition item. Personally, if my team needs me to pick it up, unless the enemy team is all late game and we have a huge mid game siege team, I will save the RG until after getting both a core Armor and MR item first. At the end of the day, normally these tanks are zoning tanks anyways (especially after the Cinderhulk nerf) and all you actually need to do is to soak up the early blows in a teamfight from the enemy backline. Even with the additional bonus 25% HP off, it's just 2500 gold for 750 HP mid game plus the passive. With all of these considerations, for your RG to be even baseline gold efficient, you need to value the active at 500 g. You have to ask yourself is that good enough, especially since your health doesn't mean as much without resists, and to me, it's simply not good enough to rush second.