I’d say that peak “can’t we just” comes with an assumption that we could just, which is quite different from this comment which comes off to me as hoping to learn why we can’t just.
I often want to honestly ask "why we can't just do X", but I feel like I end up coming off as asking "can't we just do X" no matter how hard I try to phrase it otherwise. I feel like this is a place where English grammar just doesn't give you any good options.
I’ll often ask “what are the problems with doing X?”.
It doesn’t imply that there should be a solution but you still want to learn why it might not work. Often you can continue the conversation enough to either exclude it or think some more about the problems that are described.
