Uh..uh...because it's an important part of her character? Disability representation *is* important and her being a black disabled woman makes it that much more important. There is no logical reason why they'd just take that away.
We've seen time and time again though that some of these important aspects of these characters change a lot. Major elements of Luke Cage's backstory were changed for his show. For example, Luke Cage became a southern boy from Georgia who served in the Marines and local police department.
The reasons might be logical, but I can think of many reasons why they'd take that element away.
Case in point Faith Cassidy, a major part of Rick Grimes character was his losing an arm and getting it chopped off. That didn't happen in The Walking Dead TV series. That element was changed.
Now I don't necessarily agree with it. But there are many reasons I can think of why they wouldn't do it. The Netflix shows are depicted differently than the movies. If Misty lost her arm, they'd have to figure out how to get a high-tech prosthetic. People like Misty don't have easy access to technology like that in the Netflix shows. It's probably an aspect of Misty's character they don't want to focus on for right now. At the end they wanted her to be able to walk into a club to go under cover watching over Mariah and Shades in a revealing outfit. That'd be harder if she was missing the arm or had a new prosthetic. The new prosthetic would be a whole subplot that could be difficult to figure out in the world of Luke Cage.
And I'm not sure why you're so convinced that it isn't happening. They teased it on LC? So?? Again, there is the possibility that tptb wanted to save it for another season or another show. It's not like they had a lot of time to explore it after she was injured. And they clearly wanted to set up more story for her before the season ended so the arm didn't take priority. I'm not going to read more into than that, unless/until we have real reason to believe otherwise.
Because they baited and switched the device. They started that to make everyone think that's how she loses her arm. But it ended with her being fine and her arm getting fixed. There was no foreshadowing that there were problems with her arm and it's going to be problematic later. It ended with her arm intact walking into the Harlem Paradise club. There's no indication it will be an issue again.
After this incident, where they just went through this whole process just to tease us and not go through with it, it's not happening. They touched on it by putting that incident in, but I doubt they revisit it.