To echo a comment I made a couple of months ago: "why not simply..." is a way of thinking out loud and providing an opening for someone more knowledgeable to explain what is being overlooked. It makes for interesting threads.
Pretty sure there are subtle differences between speakers of different languages once English is translated/understood.
Be careful about your tone or this will be far worse than "why not simply?" ever could be.
I’d imagine the context you use it in matters a lot, especially if you follow it up with another question or a clarification (e.g. “I’m genuinely asking”/“I’m really not sure”), but if I wanted to express the meaning you’re using it for, I might just leave out the simply, or if I wanted to it come across less confrontationally, phrase it sans the “not”, as it’s a little negative