because translucent Magazines are super tacticool, however need liiiiiiiittle more complex rendering (double sided Material for Spring, few more Faces for Follower and Base Plate)...
Spring is just a textured Mesh to keep Polycount manageable.
Magazine Case has no Interior and is closed in top to minimalize overlap rendered Faces with Alpha Blend shaders. Most of the time the Magazine will sit in a Pouch or inside the Weapon anyways, so you'll only notice looking straight down into the Magazine.