Rockstar employ, at a guess, 1,000-1,200 people. It stands to reason that if all 1,200 are working on VI it would be released earlier. They're not. Some are still churning out heists and other content for V Online. That might only be 100-200 people but it still makes a big impact.
So from wiki they have over 2000 employees split over 9 studios. As with a lot of game development they are probably using outsourcing companies as well, since certain development items would fluctuate in the amount of people needed.
I understand why you think, what you think. But the truth of the matter is that it is baseless speculation because we don't know what the critical path is for this game.
Here are some reasons off the top of my head for why it could be taking so long:
It could be that they had to build a new engine from scratch to deal with the larger map and they are just having issues with it.
It could be that they needed so many assets for this game that it is taking a ridiculous amount of man hours to complete.
Maybe they have an insane number of quests with intricate stories attached to all of them
Maybe the main storyline is constantly being refined.
Maybe they are writing a huge amount of dialog for all the NPC's in the game and it is taking a while
Maybe the game is so large and complex and is filled to the brim with bugs and it is taking a long time to fix them.
Or maybe they've had to restart development several times due to not being happy with the test builds (That's a personal theory I have for games with unusually long development times)
It could be anyone of these things or other things that I haven't even mentioned. Maybe it is multiple issues compounding together.
Your analysis assumes that throwing more people at the problem will automatically increase development speed, it reminds me of this statement "If 1 woman takes 9 months to create 1 baby, then 9 women must take 1 month to create 1 baby"
Some tasks are parallel but some are very linear.