Because depression is not incurable and most of the times it is "incurable" it's due to a lack of effort towards change by the individual. I was guilty of this in the past, if you told 20 year old me "yeah but it's OK to just give up and die if you still feel bad a year from now" I would not be here.
As mental health becomes an even more important part of young people's lives going forwards, this is not something you want them to be thinking of as an option or resolution. Society telling them yes, it's okay to kill themselves, how can you not see the effects this will have on the most vulnerable people?