“Standing on the shoulders of giants” is a saying in science. We build on the work that came before.
Same with Rockstar. Go back and play GTA III, Vice City, and San Andreas.
You really feel how these were built on the same engine/platform and how each game kinda just feels like the game they made while making the other game. If you look at the timelines San Andreas came quickly after Vice City (by modern standards at least). Imagine if they didn’t upgrade and reuse assets? If everything was to be built from scratch.
So therefore they should model everything anew again?