If wizards were incapable of shielding against fast moving projectiles, they would have more spells dedicated to creating fast moving projectiles against their opponents.
I think part of the subtext of the wizarding world is its parallels to British sensibility. You do things the way they've always been done because that's just the way of it. You don't come up with new spells, you rely on the tried and true spells that have been around for centuries.