Why does the airflow at the leading edge of the wing curve toward the trailing edge of the wing?

As long as an aircraft and the wind that surrounds it have a relative velocity, you can say that air flows over the wings.

Reason for that is either the engine of the aircraft generating a forward motion and thus creating the relative velocity with respect to the wind, OR

the potential energy of earth pulling the aircraft towards earth and thus speeding it up, again causing relative wind velocity. This is how gliders work.

A pressure difference is required to maintain fluid flow only if there is resistance to the flow. If the flow is frictionless and there is no compression of the flow, no pressure difference is required.

Imagine that the wing is infinitely thin and perfectly aligned with its direction of motion through the air. In that case the air will have no effect on the wing and the wing will have no effect on the air. No pressure differences.

Of course the situation is different if the wing is an actual airfoil shape (rounded in the front, "cupped" slightly downward, and tilted slightly relative to the motion of the wing through the air). In that case, there is a buildup of pressure at the leading edge of the wing, because the air is compressed somewhat at the front of the wing as the wing moves through it. But the increased air pressure at the front is certainly not the cause of the relative motion of the air over the wing. Instead, the relative motion is the cause of the pressure difference.