...because it's almost certainly a lie. It's because they want to better communicate the scale of Reddit to their customers - the companies on the other side of the links who they are driving content to.
It's way easier to say "Hey, we're giving you XYZ traffic, give us ABC dollars," when you have the figures in front of you rather than just upvote/downvote numbers.
It's way easier to say "Hey, we're giving you XYZ traffic, give us ABC dollars," when you have the figures in front of you rather than just upvote/downvote numbers.