I think the idea is that before the NCR their enemies mainly were other small groups with shitty weapons, and the Mojave is just kind of uniquely stuffed to the brim with good firearms, or something.

But yeah they don't really make any sense, and it's always annoyed me to no end that video game conventions demand that in a Legion ending, Caesars incoherent plan to just magically transform the Legion into a real civilization after crossing the dam and taking Vegas, basically kind of works, rather than inevitably collapse when brain cancer or just regular old age takes Caesar and leaves the Legion with no leadership in a couple of years.