Why do the people in Borderlands wear masks?

Answer 1

There are three reasons for this:

1 - The developers don't need to design or animate a huge variety of faces to add on to all the enemies, so giving them masks bypasses that need.

2 - The second reason is that most bandits in real life wear masks to hide their identity so nobody knows who they are plus, they do this to show which faction they belong to.

3 - Pandora is a hostile world full of dust storms and other environmental hazards. The masks can be protection against this, and offer minimal protection against being hit in the face.