Because they could not move. Obviously that was contradiction unless you believe dying Obito is faster than Sasuke and Naruto in their Rikudo form.- Obito and Kakashi had to obstruct the ash bones to protect Sasuke and Naruto.
- Naruto used an army of Kagebunshin to avoid being hit by the bones.
- Kakashi did kamui the bone away which would have hit Naruto.
- he slipped through Kaguya's attack, when her speed according to Black Zetsu increased.
Do you understand how the intangibility of Kamui works? He doesn't need to react at someone's attacks. The Kamui gives him the ability to overlap with physical objects and become intangible. Automatically. Obito let attacks slip through him which he couldn't see. If Sasuke teleports behind Kakashi and attacks him, his attack slips through him.
2. You can't assume that this form of Chibaku Tensei works against any object. Maybe it only works against objects he controls with his Rinnegan eye, making them to the source of gravity. Even if it would work, still don't see the effectiveness, when Kakashi can just slip through the rocks. Intangibility means he transports his physical body to the other dimension, thus, nothing works against him (also not gravity).
Plus, Sasuke's teleport technique has an interval. Kamui hasn't. His Susanoo is also stronger. Sasuke's Susanoo as well has no counter to Kamui-(Shuriken), each Shuriken being able to absorb PS sized objects.
None of these prove anything because there hasn't been one time they have been hit by the bone. And Kakashi only could go intangible because Susano'O shielded him.
It has been proven that Obito's intangibility is not automatic, Minato blitzed him a few times and he was noy intangible.
And when he does that, Sasuke ends him with Amenotejikara.
No, but his arrows counter it fine.