Because the gap between the water heater and the vent pipe serves that purpose and it doesn't need one.
A draft hood (or gap as you put it) serves a completely different purpose than a barometric. According to ASHRAE a draft hood isolates the appliance from the flue allowing combustion to take place without venting action. Since it disconnects the appliance, draft and air through the burner is completely uncontrolled. Draft hoods are designed for a specific set of conditions, which are; a vent of not more than 5 ft in height with 1 or 2 elbows. How often do those conditions get met in the field???
A barometric or draft regulator on the other hand allows for a consistent amount of air through the burner that does not change with changing conditions. Those of us who test for CO find draft hoods spilling quite often, either with higher than normal or lower than normal flue draft pressure. They are not safe under all conditions! A draft regulator is the only way to fix them. A spill switch added to the bottom of a double swing barometric will shut down the burner if the flue gets blocked.