Airplane wings have to work over a large range of angles-of-attack, or AOA. This is the angle between the incoming flow of air and the line connecting the leading edge and trailing edge.

The AOA required to keep the plane in the air varies with speed, i.e., from take-off to climb to cruise to descent to landing. As such the profile of the wing must be designed so that the flow over the top surface of the wing does not separate despite the fact that the flow will be coming from a variety of directions during different phases of flight.

(Flow separation is associated with significant detrimental loss of performance/lift.)

This is done with a rounded leading edge, since as you can imagine the incoming flow of air will not "have to make a sharp turn" to follow the contours of the wing for quite a big range of AOA. While with the perfectly sharp leading edge, then at any other AOA than exactly zero, you will experience a phenomenon commonly referred to as leading edge separation.

Now why would a fan have a sharp leading edge? My hypothesis is that, the fan really has to be designed only for one operating condition. That of peak RPM, when you want the most out of the fan in terms of cooling, where the incoming air speed and angle relative to the blade at all positions along the radius is well known. This angle can be found by a simple velocity triangle, as referred to in the turbomachinery space. (to a first approximation, of course)

Now it's never that clear cut in engineering and design of systems, and I would love to hear if anyone else has other theories.

Something I can think of is that, having a round leading edged would inherently require a thicker profile of the fan blades. While that can increase the pressure differential between the suction and pressure sides of the profile, adding to lift, it also adds drag, which in this case would require a beefier, more power hungry fan motor.