Brock was one of the best baserunners ever, but you're talking about one facet of the game. One that's not as important as the other two (especially offense). And while Brock wasn't a bad hitter at all, his offensive stats still weren't very good for a LF. Among LFers with 5000+ PA, he ranks 117th in wRC+ at 109. Behind the likes of Dave Kingman, Pat Burrell, and Roy Sievers.