Eg rarely does someone go to their boss with the thinking of "I'm not working any harder, nor am I taking on any additional work, but I'd like a pay rise anyway."
Every time ive asked for a raise its because I thought I deserved more. if you (my employer) disagree then I am happy to hear your thoughts on how I can do better but asking "why" just seems like stalling, as if you wanted me to think you didn't understand the expressed sentiment in asking for a raise.
To be honest this seems like passive aggressive management techniques to me, and I value honest straightforward employers who are proactive about employee happiness.