Because it would be out of character for Dean to approach Castiel at this point. Because the long-slow build is infinitely more rewarding. Because the characters need time to develop and come into the relationship in believable, realistic ways.
The thing is, I don’t want Destiel for the sake of Destiel. I want Destiel because I believe the narrative has been building towards it. That means I want a pay off from the narrative. I have invested time and sanity in this story. I want it to play out slowly. I want it to resolve naturally. I want Dean to address his bisexuality and come to terms with it. I want Dean to address his feelings for Castiel and come to terms with them. I want Cas to address his suicidal ideation. To address his self-loathing, and find a place of healing. I want Cas to understand that his feelings for Dean are not only welcome, they are desired. I want a crescendo of emotion crashing down around me when, after years of tiptoeing around the issue—around each other—they finally come to grips with what they mean to each other.
I want their story to remain epic. Because it is epic. I want the novel, not the short story. I want the triology, not the blockbuster. I want the writers to take the time and care these characters and their relationship deserves. Because anything else would be horribly, achingly disappointing.