fluid dynamics - Bernoulli Experiment- why is the pressure on the left greater?

Is the Pressure $P_2$ caused by a reaction force to the fluid-fluid collision, or is it the result of the collisions of the water molecules with the walls at the neck of this experiment. The part which confuses me is that $P_2$ needs to be less than $P_1 $but $P_2$ is exerted over a longer distance and the pressure exerted by fluids is calculated by $hdg$. Is there a reason why $P_2$ is less than $P_1$?