What could be the reason SPICE is outputting very different CTR curve for this optocoupler?
It is possible that the SPICE model was created for a particular version of the 4N25 that is manufactured by someone other than Vishay. Note that the results from your SPICE simulation do meet the datasheet specifications so in that sense the simulation is correct. The graphs that are provided in the datasheet are for informational purposes; the specified minimum CTR is 20% and that is the only guarantee you have.