Why does the fan rub on the shroud of a 1993 F150?

Truck might have been smacked earlier in its life, distorting the radiator supports or mounts. Lifts on a truck can also create this problem.

Assuming the truck is stock [not lifted, no accidents] and the plastic shroud is not damaged, then check the fan clutch mechanism. That attaches to the fan and disengages when vehicle speed is such that enough air comes into the radiator without the fan, thereby saving some engine horsepower and fuel. When they are either improperly installed after some other mechanical work or if they are old and going bad they can cause fan blades to wobble/rub out of normal alignment.

Change the shroud. It's probably bent out of shape from an accident or some other form of abuse.

how bad is it rubbing? those shrouds can be adjusted a hair left and right and if need be a few washers can raise it.loosen all screws holding it and see if ya can manipulate it.

is this truck lifted? a body lift will do just that if not done right