A big part of this is that the existing technology ISN'T a magic fix. It has side effects, it works differently than traditional hearing does and this requires long periods of adjustment and learning to bear with it. Literally being able to magic your hearing back to what it naturally should be doesn't have those significant downsides.

This developed because it couldn't be fixed in our world, long enough for these people to develop communities, culture, and literally their own language.

In a world where it could always have been fixed, such communities and cultures are not likely to have ever developed, since the only people who could not get it fixed would be poor, and the poor are in a bad position to gather together in groups based on their shared experience and thus be able to form their own culture.

Furthermore, people not wanting to be cured today exist in a world where there already are significant accomodations for their disabilities. It is not likely these people would be able to do this if our society had not made the collective decision to put in the effort needed to accommodate disabilities.

Yes, but that is because they've either grown up that way or have been deaf for so long that they're fully integrated into the sub culture. In a fantasy setting, deafness would be taken care of before it could influence people culturally