They dont have to.
One has rinnegan and can solo no need for training
Other can let kurama out of dog house no need to train
They still need to train to master those powers, they both haven't train with any of their powerups except Naruto who had incomplete training for KM. Sasuke usually learns to use his eyes in combat as he did with MS and EMS (a little) and Rinnegan (a little) but he didn't know how to fully use its power. Naruto needs to train to learn to use his modes as seen with KM, he didn't even know he could fly or use his TSB as effectively as Obito or Madara.
Also we do know Naruto trained after the war arc. His chakra control, taijutsu, and skills have all increased in the last. He was fighting puppets without needing clones while Hiashi and his body guards couldn't; Hyuuga's have Konoha's strongest taijutsu style as stated by Lee and Haishi is a powerful Jounin with the Byakugan but base Naruto had no issues fighting them solo and only used clones so he can chase after Hinata. He also was using FRS with 1 hand when he use to need 2 clones to help him even in SM. We know Sasuke gotten stronger from his chidori feat but that's the only one we get sadly.
who knows. Kurama looks like he's been sleeping so Naruto isn't really getting any stronger.
I already explained how Naruto as shown stronger in the Last, 2 years after the War arc, so no doubt he had some more training in the next 10 years as well or for most of the years. At the end of the last it shows Naruto doing a little training before Hinata got home with the kids so he still trained in base at that point to keep his skills sharp. Also it could be that Naruto already mastered everything he wanted too years ago so didn't need to; he can do years worth of training in a single day after all.