GTA5 would have made just as much money with a 1/100th of the marketing budget. Same as Bioshock Infinite, Last of Us, or any other huge game. They could save a lot of money on game production costs by just not doing any of that wasteful spending. We don't need to see a tv commercial or billboards for these games.
The assumption is that for every dollar you spend advertising, you make at least one extra dollar in profit to cover that advertising cost.
That doesn't always work in practice, but most companies publish more than one game, so they have more data points to look at to see how effective their marketing is, while we just assume it's stupid and unnecessary.
It's not like they throw millions at marketing just for fun. They hope that the money they spend will more than pay for itself.
If you make a game and it costs $10m and makes $10m after sales costs, that's covered its costs. Fine.
If you make a game and it costs $10m and you spend $10m on advertising and it makes $25m profit, you've made an extra $15m by spending $10m, so you're $5m better off. You have +$5m from where you started, rather than $0.
So, why do you advertise? Because it's supposed to generate more extra money than it costs. Duh?
Maybe some companies over spend, or could get the same end result with less because they have diminishing returns on their budget, but that doesn't mean advertising doesn't make sense.
You assume GTA5 would have made the same with 1/100 of the budget, because you pulled random numbers out of your ass with no basis.
Worldwide it took 6 weeks to outsell GTA4's lifetime sales (GTA5 29m vs GTA4 25m in 5 years).
Do you REALLY think that GTA5 would have sold that well if it hadn't had a large marketing budget? Since the previous data point, GTA4, indicates no, it would not have.
You also have to remember that when a game sells at launch, it sells for a higher price than if it's a 5 year old game being sold, so 29m sales right at launch is an even higher amount of money more than 25m sales over 5 years.
So, why did it have a massive marketing budget? In order to make a massive amount of money. And no, it wouldn't have made as much without the marketing budget.