We do have caustics via the PT, but if I had to guess it’s because the engineers are handling other features first, as caustics are an incredible phenomenon but probably not as essential to most users as simply getting lumen to run better, or have less noisy GI.
Nvidia does, for the record, have real-time caustics in Unreal. I did a pretty big review of their tech a big ago, and my bottom line is that it’s a technical marvel held back by poor UI (although that has been partly rectified to my knowledge).