Why has she not changed at the end of Suruga Monkey?

Question

Why has

Kanbaru not regained her human arm

by the end of Suruga Monkey in Bakemonogatari?

If the Rainy Devil is gone, why is she still the way she was when it possessed her?

Answer 1

So that this question can have a real answer, I'll expand my comment.

At the end of Suruga Monkey, Hitagi comes in and interrupts the fight between Koyomi and Suruga, announcing that if Suruga had successfully killed Koyomi, she would have hunted down and killed Suruga, adding a serious note to her joke in Part I that she wanted to be the one to kill him.

Koyomi then explains in the voiceover narration (translation from subtitles on the horrendously overpriced Aniplex USA Blu-ray):

If one strategy was for me to overwhelm the demon, thereby making it impossible for Kanbaru's secret wish to be fulfilled, then notifying Senjyogahara that Kanbaru was trying to kill me, thereby making it impossible for her surface wish to be fulfilled, was another. Moreover, Senjyogahara swore in front of the demon that if I died, she'd kill Kanbaru. Not knowing wouldn't hold up as defense. For the Rainy Devil, the outcome had already been determined. I can't believe you were able to read its thoughts like that, Oshino.

Suruga's secret wish was to kill Koyomi out of jealousy that he was with Hitagi and she wasn't. The Rainy Devil tries to help her fulfill that secret wish in Part I, when she attacks Koyomi on his mountain bike as he rides home from Hitagi's apartment. Koyomi planned to fight Suruga until it was clear that she couldn't beat him, making the Rainy Devil give up.

Suruga's surface wish was to be with Hitagi romantically, but Oshino realized that if Hitagi knew that Suruga had killed her boyfriend, she wouldn't be too keen on hooking up with his murderer. Sending her into the middle of the battle made it impossible for the Rainy Devil to get Suruga and Hitagi together by killing Koyomi, giving it no reason to help Suruga kill him. Fulfilling the secret wish was only a viable way for the Rainy Devil to complete its contract if that also fulfilled the surface wish, but once Hitagi knows, killing Koyomi directly counteracts the fulfillment of the surface wish.

To fulfill its contract and leave Suruga, the Rainy Devil has to grant her wish, but it fails to do so, made clear when Suruga confesses her love and Hitagi responds in her usual blunt fashion:

Suruga: I'm in love with you, Senjyogahara-sempai...
Hitagi: Oh? I'm not really in love with you. But will you stay by my side anyway?

So the Rainy Devil can't leave, because it hasn't fulfilled her wish, but it can't fulfill her wish, because the only power it has is sending her into a violent rage while she's asleep and it now knows violence can't solve this. It goes dormant.

Oddly enough, in the final scene where she goes to meet Koyomi, Suruga says, "The demon may have left me, but in the end, my arm didn't go back to normal." In-universe I can only attribute this to a misunderstanding on her part, because in Hanamonogatari, which takes place chronologically last in the series (beware spoilers below):

Suruga finally does lose the demon to her old friend Numachi Rouka, and her arm goes back to normal. In the beginning of the Hanamonogatari novel, we see that she's been duct-taping her monkey arm to a pillar every night and anxiously checking the newspaper every morning to make sure she hasn't gone out and hurt anyone in her sleep, so she doesn't seem to be so sure anymore that the demon is gone.