In breaststroke, why head pops vertically so much?

As obvious from watching breaststrokers in the pool, their heads and upper body seem to have more vertical movements than forward movements?? The scene is: up, down, up, down,... Some swimmers seem to be even looking down at the water under their chin, rather than looking forward. Isn't that a lot of waste of energy? Why not flatten and extend your body more and keep parallel to the water so most of the movement is going forward instead of vertical popping? :confused: There must be a reason behind this since a lot (maybe most) of people do so?