I think it is because kishi want it this way.
If sasuke solos kaguya it wont really be team work, he already solo'd madara who was the main antagonist at that time.Sasuke doing all the work might displease naruto fanboys.
So6p implied they both need to work together so maybe sasuke is doing this because of the respect he gives to so6p.
Honestly, It is getting really boring now... sasuke is literally babysitting naruto. While the power was distributed between them equally, its clear that sasuke is on a whole different level than naruto.