Because its a complex disorder and everybody is different.
CBT works, lots of things work but they are tools. Sometimes people have toxic shame or trauma to deal with as well as very low self-esteem. They may live in an isolated community where there is a lack of resources or help to deal with their SA.
They might be dealing with other things and putting themselves last. They might not have any support.
At the end of the day, its the energy, willingness and motivation to get out of a SA routine and comfort zone, its the managing of stress, its the self-esteem, and the identity issues, sometimes its the lack of having someone to give you a push. Its many things why people have trouble getting over it. But i think its more of managing it than getting over it.