Wouldn't he have been VERY famous?
Ok, some of the things he did he didn't get credit for, such as the Elvis dance or the smiley face.
And if he only did one or two of the things he got credited for then he could've easily been forgotten.
But a notable war hero, a football champion, a ping-pong champion who was so big he became a Cold War era star, a person who got to meet several Presidents, appeared on The Dick Cavett Show next to John Lennon, held a speech at the March on the Pentagon rally next to Abbie Hoffman, was all over the news during the long period when he was running and running, Bubba Gump Shrimp Company & Apple Computers owner and millionaire.............would've been fcuking famous!
The press loves an overachiever, and this guy had achieved more than anyone else even if we ignore his 75 IQ. If he tells his life story to random people on a bench, then he would share it with reporters as well. He would be bigger than Helen Keller and Stevie Wonder combined.
But no one recognized him, except for the guy who laughed at the thought that Gump would be the owner of Bubba Gump Shrimp Company.
Now, of course, Gump's life story and achievements in itself are extremely unrealistic, but they follow the rules set by the movie.
No explanation is given to why he isn't more famous. The bench scene is supposed to be in '79 or '80, so you can't use the time excuse.