Lol Just because he didnt doesn't mean he couldn't.
Obito could've kamui'd to every hidden village & captured all the jinchuuriki by himself, why didnt he? Because then the manga would have no plot.
Use logic, obito phases underground & appears behind itachi, Then what? Obito's chakra is undetectable until he becomes solid. In an actual fight between the two obito simply creates an explosion big enough to hide his intangibility. Itachi has literally nothing on obito & kamui, especially with no info on it.