Why do only the 3 4.0 lvl 70 dungeons drop weapons?

[Sekkei]

I thought Shinryu Normal was never nerfed? I at least don't recall any nerfs unless it was week 1 or 2 of 4.0.

No, he hasn't been nerfed at all unless there's a patch note I'm missing. I'm pretty sure he's still the same as he was when released. If you want to think of it as a nerf on a sort of unrelated thing, Shinryu Extreme was not added to mentor roulette when it was taken out of raid finder and moved to duty finder on patch 4.2. I believe that's the first primal SE probably has not done so for it.