of course, Obito was actually in a rush at the moment, so he can't afford wasting time, or else he could've did the same trick he did against Danzo's body guards, fake the counter attack(grab in this case).
BTW Obito can't choose which part of his body gets sent to Kamui dimension, he can only activate intangibility, so whenever an object overlap with his body in the real world, the overlapped part gets sent into Kamui dimension, he can't keep a part of body tangible while the other part intangible.