Why is it that there are people who believe you have to know ASL to be a complete deaf person?
I don't think anyone here is trying to argue that you need to know ASL to be a complete deaf person. I think the claim is that you need to know ASL to be a complete Deaf person. That is a very real and true statement and I don't think it can be debated very easily. While it is true that not all deaf individuals will want to embrace Deaf culture and community, many deaf individuals will feel the need and drive to align with their culture.
This drive to find socialized identity is true of all people. If you are a Christian, you may seek out a church and people who share your religeous views. If you are part of a visible or ethnic minority, you may seek out people with the same ethinicity. If you are d/Deaf, you may seek out other people who are d/Deaf. The key word is may!
The funny thing with Deaf culture is that it is more often than not a culture that is only truly accessible by one member of the family, in the case of this discussion that person is the child. While I cannot possibly understand what it is like for a parent to experience a cultural divide with their own child, I can empathise from the side of the child.
I know I didn't make any distinct effort to reduce the cultural divide between me and my parents. If anything I went out of my way to enhance it. My parents started signing with me as soon as they found out I had a hearing loss. I was language delayed and didn't speak until I was around 3. I signed until I started speaking and once I started speaking my vocabularly blossumed. I went from from being language delayed to completely cought up in my development in the matter of a year. By the time I was 7 my SLP was telling my parents I was now ahead in my language development. No one signed with me anymore and I didn't feel a need to sign with anyone either.
As time went on and I grew into a young woman I realised my differences more. No one was protecting me anymore and I liked that but suddenly the world became scary and I was isolated. I wasn't surrounded by people who knew how to communicate with me and I was really alone. I chose to seek out connections that aligned with all of my socialized identities. I found other d/Deaf people to be my primary group of friends. I was able to do seek out that community because of my grasp on ASL. I am not saying that this is a universal experience for all oral deaf children. I have friends that were raised oral and remain oral to this day - with no drive to learn ASL. I am simply speaking of my experience.