OK, so the two most powerful wizards in the world can create non-protego barriers, so what? They still can't attack through them, and if they drop them they are still shot. HP wizards seem to be basically limited to either attacking or defending, but not doing both at the same time. Their defenses can stop pretty much any number of arrows, but if they drop them for long enough to attack, they'll sprout some feathers in their internal organs. It all comes down to the number of actions that can be taken- HP wizards can cast spells probably somewhat faster than an orc can nock and fire an arrow, but are so vastly outnumbered its functionally irrelevant in all but one on one combat. Four or six orcs can simply act more often then the wizard can, and with a roughly comperable degree of lethality (spells more deadly but less likely to hit), and so can either score a kill or force the wizard to flee, while taking minimal losses- even if the wizard kills 5 out of six, as long as the sixth orc manages a kill, its still a win for Sauron.

As for Gandalf running off the Nazgul, at that point Gandalf was the second most powerful being in Middle Earth, and the most powerful force for good, a literal walking demi-god of good in fact, who turned the Nazgul with a radiant blast of holyness. Probably not something that evil human wizards can pull out of their hats...