I had heard about this giant JSON from friends in the GTA V modding community. OP's idea of what it is used for is right. My guess is that this JSON was quite smaller when the game released and has been increasing in size as they add more and more items to sell in-game. Additionally, I speculate that most of the people with the knowledge to do this sort of profiling moved on to work on other Rockstar titles, and the "secondary team(s)" maintaining GTA Online throughout most of its lifespan either didn't notice the problem, since it's something that has become worse slowly over the years, or don't have enough bandwidth to focus on it and fix it.

It's also possible they are very aware of it and are saving up this improvement for the next iteration of GTA Online, running on a newer version of their game engine :)