> I wonder if thats because, if you let engineers do both, you end up with a mess and accomplish nothing
No, it is because an Engineer (with technology selection blinders on) will jump to a What/How before the Why actually lands in a conversation.
The why will be recorded as "what the customer wants", we jump from problem to solution too fast.
In fact most of the times the product org only provides a "What" to the engineering team which is where most of the problems are buried & plays it close to the chest with the "Why".
I was an engineer reporting to the CPO in my last job and my entire job was to diffuse the "Why" down to the engineering org, so that they felt purposeful about the problems they were solving & to push back on the What if there's a better solution possible. But I didn't get to redefine the "Why" part of the question, even though I really wanted to do it to make more progress in the directions I was already headed.
Once you got "what", then you get to "how", "who", "when" and usually "how much?".
The message usually bounces off the end of that and goes back, to the customer who's eventually going to solve the "how much" problem.