So you might think that the federal panels would have done a good job of redistricting. You would be wrong. In almost every district, their handiwork shows signs of partisan gerrymandering. It makes one district safe for one party and the adjoining district safe for the other. The truth is that both parties want it that way.
Incumbents desire safer districts.
So when it's time to redistrict, the people doing it simply draw a line like a lasso and rope the Democrats out of the Republican district next door, which of course makes that district even more Republican, and vice versa. The traffic is two-way. The judges go along with this because both parties do. The parties drafted the plans that the judges often only tinker with. The result is that the incumbent cannot be defeated, and the voters in effect are disenfranchised.