He had indeed already taken a physical form, at least to some degree, for he was the Necromancer mentioned in the Hobbit, whose realm bordered upon Mirkwood. But at that point he had not regained a large portion of his power and worked mostly in secret.

He also greatly feared two things. First, the return of Numenor's heir, Aragorn, and the sword reforged. Unlike in the movies, in the books Aragorn left Rivendell with the sword reforged from Narsil and did not hide who he was, though he didn't travel openly as such.

The second thing he feared was that his agents had not found the One Ring. While there still remained the chance his enemies had it or might find it before he did, he could not risk exposing himself directly while he was in his much weakened state. Had he gotten the Ring, he would very likely have not hid in Barad-dur while the Nazgul lead his armies against Middle Earth.

He had indeed already taken a physical form, at least to some degree, for he was the Necromancer mentioned in the Hobbit, whose realm bordered upon Mirkwood. But at that point he had not regained a large portion of his power and worked mostly in secret.

He also greatly feared two things. First, the return of Númenor's heir, Aragorn, and the sword reforged. Unlike in the movies, in the books Aragorn left Rivendell with the sword reforged from Narsil and did not hide who he was, though he didn't travel openly as such.

The second thing he feared was that his agents had not found the One Ring. While there still remained the chance his enemies had it or might find it before he did, he could not risk exposing himself directly while he was in his much weakened state. Had he gotten the Ring, he would very likely have not hid in Barad-dûr while the Nazgûl lead his armies against Middle-earth.