Why do NBA Superstars seldom guard each other...seems like they are always being guarded by the scrubs (role players)?

Because a lot of the players considered "superstars" are actually pretty one dimensional, and a lot of role players are better than them on defense. Harden is the obvious example, but Damian Lillard and Russell Westbrook are almost as bad, and Durant and Curry are both average defenders for their position too.

It's worth noting though that players like Kawhi Leonard, Lebron, CP3, Draymond Green, and Paul George still exist that still dominate on both sides

The basic math most teams use is: 1 Defensive Action = Minus 2 Offensive Actions.

So if a Superstar plays all-in on D, he usually struggles to come back on Offense the next plays, some teams sacrifice that, but most will bank on Offense only, and keep the Defense for the role-players.

Thinking back to Kobe Bryant and the Lebron James and Dwayne Wade days ...Larry Bird and Magic Johnson did at times but it was mostly other players. Is it due to the exhaustion of having to play both ends of the floor with maximum effort burning them out and making them tire out quicker? Is that the only reason? I remember also Clyde Drexler and Michael Jordan never matched up one on one in games neither and so many countless other NBA superstars in history.