Do me a favor. Head over to Google.com (in a new tab of course) and type "gta v mods" into the search box. Navigate to whichever mod site you prefer, and click on the "featured files" tab, or its equivalent. What do you see?
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Lots and lots of cars.
Not just any kinds of cars, mind you, but real cars. 3D models of existing, actual cars from real world companies.
These mods have seen massive popularity amongst players and this shows one clear message: GTA V players want real cars in their game. This could also be seen in previous GTA games, the PC versions of which saw countless real car mods as well (171 pages of results right here!). So, if it is so clear what the players want, why won't Rockstar add real car models into Grand Theft Auto?
The answer is actually quite simple: the licensing fees are extremely expensive. Granted, GTA V had one of the biggest budgets in video game history, second only to Destiny. But when you play the game, you'll see that every cent was well spent (well, almost every cent - GTA V had pretty solid marketing going on). This means that adding licensed vehicles would result in GTA V being much less a game then it is now.
What's more, considering the wide range of vehicles available in GTA V, a heck of lot of licenses would be needed. Clearly, doing some licensed cars and some fictional ones would be half-assed, so they either go all the way or don't do it at all.
Obviously there would be multiple makes of cars, and several different models. So if you add up all those license agreements, with all the work hours in negotiating with so many different entities, and the consultations and whatnot, you'd probably come out with a game that costs three or four times as much to make. Plus, I highly doubt real cars as a feature would bring in that many more buyers, considering there probably aren't many people left who would buy GTA V but haven't.
What's your stance on future GTA games having real cars? A nice to have, a waste of money, or a must-have?