They did do LATD and BOGT but they was still based on Liberty City and basically expansions. Where as with the GTA III series you had Liberty City (GTA III), Vice City, San Andreas, Liberty City Stories and Vice City stories before they moved onto the GTA IV series but instead of doing what hey have done with III they have moved on to V. It's not a problem I just expected more and was dissapionted, hopefully GTA VI will be Vice City again
They milked a dead cow with GTA3 though. DLC expansion packs is all you want really, and only for a limited time. Otherwise they will never develop better game engines as all their resources will be tied up doing crap spin offs.
I don't think they milked it with VC and SA as they were fully fledged games in their own rights but LC and VC stories were for the PSP and just ported over and tbh more like a late expasion to old games and I think it was only done because of poor sales on the PSP versions. But thats going back to the PS2 days when development of new games engines and the like was slow compared to what its like now.
Saying that I've read that GTA V is running the same engine as GTA IV just with improvements (which originally came out on R* table tennis and all games since and coming like Max Payne). With no new engine it does make me wounder why it wasn't GTA IV: SA as there's no new engine just an upgraded one so that makes it the same as the differences in the GTA III series.
End of the day regardless of the numbering/series its still a GTA game, I just hope it has a good story, loads of easter eggs and is fun. As much as I liked IV its far from my favourite.