gerrymandering has helped create a huge number of districts that are fundamentally safe for one party. This is sometimes done by a dominant party in a state “packing” the other party’s districts to limit its chances in other districts. Other times it is done by an unholy alliance of both parties to keep all incumbents safe.
This is very much so in the #NY26 & #NY27 Congressional Districts. Rep Brian Higgins’ NY 26 is heavy Democratic and the NY 27 district seat held by a Republican, Chris Collins. Both districts were redrawn to favor each incumbent.