Briefs because our balls get in the way and our sweaty scrotum sticks to our legs. The leg bands of the briefs sit up in the crease and separate the sack from the thighs.
Boxers because some folks don’t like restriction and want airflow to our sweaty balls. Also they come in more fun patterns.
Because that’s what our parents bought for us as kids. It’s not an important enough part of our wardrobe to change if it’s working for us. No one is supposed to see them in public anyways.