The jockey (upper) pulley on a modern indexing rear derailleur has to "float" sideways a little to make shifting quick and quiet so just any pulley won't do.
Early replacement pulleys were fixed and provided touchy and unreliable shifting when substituted for the OEM ones. I expect current aftermarket pulleys have been redesigned to provide the needed float.
That said, the factory pulleys work so well and last so long I don't see any use spending more money to replace them.