Johnson and Moss both have more physical ability and make mediocre QBs look good by out playing multiple defenders on poorly thrown passes. If Moss played more of his career with Tom Brady for example, his best seasons would all be better than any of Rice's best statistically season.
Rice was a great receiver, and probably had the most longevity of any receiver ever. But he gets over-rated based on career achievements playing on all-time great teams, who would have still won a lot without him. Same argument as when people claim Favre is the best QB cause of all his total records. He just holds more records cause he wouldn't retire and played poorly into his 40s. If you go by season averages over their careers, it's a different story.