You can't knock in runs when there's nobody on base. Indeed, nearly every Triple Crown winner has played for a winning team and on an offense ranked in the top three. You shouldn't lose out just by playing on a terrible squad. If we wanted to measure how fast you knock guys around the base paths, we could use OBP with runners on base. But this is flawed, too.
Good hitters hit well when men are on base.
They also hit well when men aren't on base. Remember, every out is precious, not just ones with men on base. Some players get walks and singles, and some hit doubles and homers. SLG is total bases divided by at-bats. This stat nicely complements OBP, and in fact many people use the two in tandem.