Not eating prekilled meat and processed meat allows you to be part of a social movement and gain status. Not killing the meat personally doesn't.

Also, people do things based on how they feel, but don't like to be reminded of that. If the particular rationalization for doing what they feel happens to include an ethical claim, even if it is just a rationalization and they don't understand any theories of ethics, they will avoid a broad enough category to convince themselves that they really are doing it based on ethics.