The 777 was the first airliner to be entirely designed on computers and thus the wing was as good as it could be got at the time the design was frozen for production. Look at the 787 - far more recent technology, designed on a compter, but still the wing does not have/need winglets!
As a general rule, winglets only seem to appear on designs that are being updated/improved i.e. tweaking older designs to improve them without going through the expense of a complete redesign and certification.