Point-guards in the NBA blow by the opponent lots of time. "Keep him between the knees" is easier to be said than done.
Point-guard defense at the NBA level is a lot about delaying the first pass on the offense, take away the middle on the penetration (forcing baseline, for example), control the passing hand, deny the easy passing lane to the post, etc, not exactly about "staying in front" all the time. That's basically impossible, at least since the hand-checking was prohibited.