Because the actors contract calls for "x" amount of face time. I'm guessing. Otherwise why would this happen so often. It's a bit cliche now. The first time I really noticed it was for Batman Returns when he rips his cowl off. Since then it's just gotten worse.
Allegedly the new Dredd movie is going to buck that. V for Vendetta is the only other one to stay true where needed. I've read Urban is masked the whole time.