Todd H, I can answer your question, and it’s all about deep flanges, manufacturing tolerances , and cost to build.
LGB started it and USAT and Aristo folllowed.
Since everyone started by copying LGB with track that could interchange, the issue continues. Been this way forever, but starts to show up on longer trains, and switches, and when you try to back a train.
Also long story on truck mounted couplers, overly deep flanges, etc.
Track and wheels and gauge and flangeways are a system, and all need to work together. What comes from the factory can be greatly improved, but you normally cannot just change one part of the system without changing the other part.
You can make things work a lot better by following the “target values” in the NMRA specs, but you will find that the switch flangeways will need some shimming to bring them to specification.
If you do this all, everything works a lot better.
Yes, you have been running fine for a while, but over time things can change/wear/move.
I’m only recommending what I know will work, and make things better.
Greg