Surely "Why don't you just... ?" is an exceptionally good phrase to use. In practice people mean "Just do ... !", which is very different. The why question, however, gets to the heart of an issue, it's a short hand for "The obvious solution appears to be that ... but I imagine you tried that and have a reason not to do things that way, what are those reasons?". It's a direct learning-centred enquiry that ekes out the kernel of complexity of a situation relying on the wisdom of the person it's aimed at.

So why don't you just use the phrase "Why don't you just ...?"?