User blog comment:Hellhamer/Captain America: The Winter Soldier Starts Filming/@comment-1687414-20130409025235/@comment-4651179-20130410195149

It's because Hollywood stars want to show their pretty faces to the camera.

"I just defeated my enemy and I have to crash a plane! I'd better take of my mask with no reason!" - Steve Rogers in Captain America: The First Avenger.

"I will attack this human from behind and take his mask off, muahahahahaha!" - Chitauir soldier who attacked Captain America in Marvel's The Avengers.

Or the same reason why both Captain America or Iron Man appeared without mask/helmet in 99% of Marvel's The Avengers' posters.