Uchiha Itachi didn't know that.
Although Orochimaru had indeed left some chakra and even traces of his soul inside Sasuke, half of that was due to the Cursed Seal, while the other part was because Sasuke had previously "devoured" Orochimaru's soul. To put it simply, during that earlier confrontation, Orochimaru had lost in a head-on battle. When he tried to use Living Corpse Reincarnation to forcibly take over Sasuke's body, Sasuke had instead taken the opportunity in that spiritual illusion world to awaken his Mangekyō Sharingan and devour Orochimaru in return. Even the Three-Tomoe Sharingan in the original story had been able to restrain Orochimaru's soul to a great extent.
In this life, the stimulation of his soul had only made it more so.
On top of that, Sasuke had consciously guided his own mind.
Sasuke had awakened the Mangekyō Sharingan outright.
Under the scrutiny of those "evil eyes",
Orochimaru's soul traces had nowhere to hide. Sasuke had completely absorbed the most essential part, then used his Mangekyō Sharingan to eradicate the remnants. In other words, unlike in the original story, this life's Sasuke had no trace of Orochimaru's consciousness lurking within him. The tiny remnant of his soul was merely what Sasuke had revealed on the surface. Orochimaru, as a person, had undoubtedly perished. Unless someone used the Cursed Seal to reverse-trace the inscribed technique, as in the original story, there was no way to revive him. Without that, even Impure World Reincarnation might not be able to summon Orochimaru—the most important soul medium simply didn't exist!
But Itachi didn't know that.
Based only on what he knew of Orochimaru, and the familiar, strange aura he had sensed from his younger brother before the battle,
Itachi assumed Orochimaru still had a backup plan. Naturally, that led him to make