Olympic gold medal-winner (in the 1992 4x100-meter relay) James Jett played for the Raiders for 10 years as a receiver. He had a very high yards-per-catch average (he was on the right end of a lot of long bombs), and his career highs were 46 catches in a season ('97) and 882 yards ('98). That’s not bad at all, but his speed was his biggest attribute and he was never even the best player on his team.
Michael Johnson and Carl Lewis were fast, no doubt, but to be that fast, you can’t be very muscular, which would be a major problem in the NFL. A runner has to be strong enough to bang people around and knock them over. Also, sprinters pretty much just run straight ahead, while running backs have to juke and dart all over the place.