I tend to interpet "Why don't you just..." as "I am not seeing what the problem is here, and I'd like to understand it so I can help you. By way of enlightening me, can you explain why you can't just do _____?"


I say "Why don't you just..." all the time, with more or less this meaning, and I know a lot of people who say it to me very often, with more or less this meaning. I'm usually grateful to be asked this, because it gives me a chance to articulate my problem more clearly.