But apparantly a significant redesign of the metal 777 wing would be needed to accomodate the raked tips.
The 777-300ER/200LR have raked wing tips (the original 777 didn't). For the 777X, the wing span is long enough that it wouldn't fit in the same gates as the current 777, so the decision was made to go with the folding wing tip.
The folding wingtip offered on the original 777 was inboard of some of the movable aero surfaces - that caused major headaches. No moveable surfaces on the foldable surface on the 777X.
The Boeing Aero types claim the raked wing tip is more effective than winglets, but it necessarily increases the wing span - that would be bad news for the 737 for the gate size they often use, so they stayed with winglets to get the aero improvement with nearly the same span.