Why is airflow forced to be speed up on the upper surface of an aerofoil?
Airflow isn't compelled to do that by some edict.  It just does it.  &quot;Why&quot; isn't all that meaningful, compared to a theory or formula with predictive power.
Bernoulli says that, under certain conditions, a fluid's speed and pressure have a certain relationship.
How a fluid flows past a obstruction, and what net forces and torques it exerts on that obstruction, aren't because of Bernoulli.
The closest we can come to a reason for the fluid's behavior is the Navier-Stokes equations.  But that's still a model, a simplification.

For an airfoil in conventional steady flight, it's actually more useful to say the reverse:  because the airfoil has been carefully chosen to generate lift under those conditions, the pressure above must be less than that below, so Bernoulli predicts that the flow above must be faster than that below.  And we can confirm that with CFD simulations or with onboard sensors.