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I find that untrue. Every material we use for making furniture can be bent or cut along a curve. It just doesn't make sense for certain applications. For example a curved bookshelf is nonsensical since a row of books cannot be curved. I have to place the books a little apart if I want it to conform the shape, but that is waste of space.
A couch could be fine curved, or a kitchen counter. But that couch could mostly be used for sitting, sleeping on it could prove for great many people uncomfortable.