I mean inside the red zone, where teams are more structured. Obviously no spine player ever 'only' attacks one side of the field, but teams have a structure and it usually involves one spine player playing between two others. Mostly it's the fullback and five-eighth who are split, which allows them to build familiarity with their edge. Yes there are exceptions:
Storm: Hughes (R) and Munster (L) are split with Papenhuyzen floating both sides.
Sharks: Hynes plays very central with Trindall and Kennedy both floating.
Seagulls: DCE plays in the middle with Brooks and Trbojevic floating, but Brooks' highlights are overwhelmingly on the left while Trbojevic's are on the right.
Broncos: Reynolds plays on the right with Mam on the left and Walsh floating. They prefer to shift left.
Bunnies: I can't work out what their structure is just from watching. It probably depends on how Latrell Mitchell is feeling on the day.
Eels: our structure is up there with Souths'. Last year we were generally Gutherson (L), Moses (R) but this year we are all over the place.
But for most teams it's the 6 and the 1 who are usually split left/right, when they get closer to the opposition tryline, as I said:
Dolphins: Tabuai-Fidow (L), Nikorima (R)
Panthers: Luai (L), Edwards (R)
Roosters: Keary (L), Tedesco (R)
Raiders: Strange (L), Rapana/Stewart (R)
Bulldogs: Burton (L), Taafe/Tracey (R)
Cowboys: Dearden (L), Drinkwater (R)
Dragons: Flanagan (L), Sloan (R)
Warriors: TMM (L), CNK (R)
Tigers: Galvin (L), Bula (R)
Knights: Ponga (L), Gamble (R)
Titans: Brimson (L), Boyd (R)
If you looked at a heat map it would show them concentrated on these sides of the field. That doesn't mean they never go where they see an opportunity, but if all the playmakers overload to one side and the play breaks down, there is suddenly no merkin ready to go for a quick shift to the other side. So it makes sense for them to remain where they're 'supposed to be'. There is also the fact that not every team gets into its red zone shape exactly 20m out from the tryline. The teams with the best edges tend to get into attacking shape further out while the teams with weaker edges do it closer to the tryline. Because they all play to their strengths, and their strengths aren't all the same.