Because if you go by that metric, he's absolutely a Hall of Famer. He's 7th all time among 2nd-basemen in W.A.R. at 75, which is even better than Sandberg, and his peak 7 seasons is even with Sandberg. The difference is that Sandberg has 5 more All-Star appearances and 6 more Gold Gloves. And Sandberg had a couple of real eye-popping seasons that everyone remembers, whereas Lou was just consistently very, very good.
The catch with that statistic is that it places such a premium value on playing the 2nd Base position that it can inflate the value of guys at that spot relative to 1B or RF, who conversely really need to have huge offensive statistics to be valued as heavily.