Getting everyone in a room can still lead to the wrong design. Situations change over time and no matter how good you try to be up front you can be faced with situations that demand change and make your original design useless. I certainly have. It is often the case that there isn't anyone in the world who knows all the details or that "everyone" is 1000s of people and beyond that there are just mountains moving in your business and you have to move with them.
Scrum makes you manage your work regularly, see early when it's going wrong, adapt to get back on track. It gets your whole team involved and you end up with solutions that were thought up by your more junior team members or people working on a totally different bit of code.
Architects are useless in waterfall because they can be long gone by the time it becomes clear that their architecture is a total fuck up.