But then it wont have super mudents and radors to fite
In order to set a game in "civilized" lands like NCR territory, you would still need to have some sort of conflict that motivates the story. Whether it's social unrest, a civil war, an invasion, criminal gangs up to no good, or whatever you would have something to fight against. I doubt a video game set in peaceful territory in which everything is hunky dory and people go on about their lives would get signed off on for an AAA budget.
If they want to do a game set in the Boneyard, they probably have some notion of what the central conflict would be.
The Gun runners use the weakness of the NCR as an attempt to take over. They have the guns and men to do it.
Why? What would their motivation be?
Easy, to increase their power and possible market base. As a separate country they could sell guns to anyone, no matter who is enemies to who. Because in NV they can't sell their gear to the legion or the various raiders because they're enemies to the Strip and NCR.
Getting into a full-scale war against the armed forces of the NCR would be a stupid business decision. If the Gun Runners took over the NCR by force, it would still make no sense to sell weapons to the Legion or raiders, who would still be determined to conquer their territory, or steal their stuff, respectively. Plus they would then have to manage the state. Keep the roads clear, keep trade, water and power flowing, organise defence of the borders.
If all they want is to sell a few guns to tribals or raiders, they should do it under the radar, or buy off a senator or two, not declare war on their own country, in which they already have a highly lucrative market. Even the van Graffs aren't
that crazy.
I don't mean straight out war, but remember, the NCR is collapsing. Why not sell it to various rebel and gangster groups to quicken their demise. It's not like the NCR is in ANY position to fight the Gun Runners and the Legion. Not all raiders are like the fiends, many like the Powder Gangers and Khans are quite willing to do business, and so are the legion. Caesar isn't an idiot. He can and will reign in his men if fighting means losing business and causing lots of damage to his own force. Yes and? Just because they sell guns doesn't mean they can't control a small state. It's not impossible. You don't have to have a grade A leader diploma or a country runner to run a country. Remember the Gun runners won't need to make a conventional state like the NCR.
Declaring war and possibly destroying the NCR opens up the market for various groups starting out, instead of being limited to one mega state that can force down the prices because they can threaten the GR. With the legion on the horizon and the NCR falling apart, they can move to the public and take up a monopoly of gun making, also regulating the prizes as no one can threaten them. That's a great deal.
As Walpknut said, the NCR isn't collapsing. Its problems are far from insurmountable.
The gun runners are an arms manufacturing company. They rely on the infrastructure of the state to survive and thrive. Supply chain, raw materials, educated labour, prosperous markets, a pretty much exclusive contract with the NCR military. What can tribals and raiders offer that would make it worth destroying the NCR for? A few caps? Some animal skins?
Caesar is an idealogue. He is not a businessman. The Legion do not appear to trade with outsiders. They are not concerned with doing business or making money, they are concerned with conquest. They are losing men to the NCR, and they certainly aren't doing business with them. They have already been beaten at Hoover Dam once. Not even an attempt at peace. I can hardly see that changing if the NCR was replaced with the Gun Runners.
In any case, I can't imagine the gun runners are in any position to take on the NCR army in a fight. They may have lots of guns and money, but in terms of numbers, they are going to be tiny. Private security guarding their factories, caravan guards to escort shipments in hostile territory. They don't even have the numbers to keep energy weapons safe from the van Graffs.