I suspect it is because the regular content updates that Rockstar has released for GTA Online have led to them earning nearly eight billion dollars from their players over the past nine years. They’ve earned almost a billion dollars per year from GTA Online for the past several years.
Also, GTA games require a huge amount of content and have a much longer dev time than most other titles. The size of the team for each GTA game is absolutely massive. GTA6 has been in development for quite some time already and will be launching on the new generation hardware. That’s going to require some time to build everything out to the levels they want.
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