Witness implies more thought, and therefore more attention. Note the English root witan (to know). It implies both the sensory impression and understanding of some apparent phenomenon.
Watch is more of a duty or chore, which may or may not be successful, (as in "fell asleep watching TV"), and may not require full attention, or even the outward senses, ("I keep a close watch on this heart of mine..." - J. Cash).
None of the suggested words watch, look, witness, spectate when put in the defective sample sentence would make it correct English:
Skilled sculptors from around the world participate in this month-long
event while millions of tourist come to ......... their incredibly
One may witness an act of creation, or a creation, but not a set of objects. To "witness their... creations" would imply the thing being is made is the sculptors, as objects of manufacture (i.e. robots or androids) perhaps.
To watch implies the watchers might be there to guard the statuary.
Less specific verbs that would fit: see, view, or behold.