Wikis only list the largest locations, not all of them.The gamepedia article about it doesn't say we can find maize in the wilderness, all the locations are either from vendors or from farms/settlements/previous farms:
http://fallout.gamepedia.com/Maize#Locations
Sure, that's possible, but its never stated. The biggest problem is that NV's history doesn't make any sense, and the main problem is House.Okay, one last reply. I gave an answer (but I acknowledge that there are gaps in lore so excuse me for making an educated guess) but I see only see nitpicking from you so let me reiterate my answer. One could theorize that there were smaller farms and settlements outside the Strip since the region was not as badly hit. The outlying area seemed fine, save for regions with extensive amounts of mutant animals, and there could have been small villages before the arrival of the main factions that would have existed but not lasted due to the various hostile tribals of the old Strip. Like this village:
http://fallout.wikia.com/wiki/Tribal_village
Don't forget, the wall of the Strip was constructed on House's orders in recent years so it would have been easier to obtain supplies from areas outside of the Strip.
There have been places where farms could have been made but in New Vegas, the setting has reached a point where those places would have been taken over by the factions like NCR Sharecroppers.
House went into a coma during the war because his systems failed due to lacking the platinum chip. He woke up from that coma in 2138, 4 years before Shady Sands was even founded, and then produced to do literally all of nothing for the next 136 years for reasons that are never explained.
House claims to want to rebuild humanity, but then did nothing to try to organize the people of Vegas for over 100 ears until the very last minute. House could have become the NCR, instead of needing to leech off of him.
The problem is only further worsened by the sheer improbably that a large group of people, in a city that was untouched by the war, and that had a massive clean water source, failed to accomplish any sort of actual civilization in 200 years, despite the fact people on both the east and west coast did more in far less time.
Everything about the Mojave's backstory is basically just one giant contrivance to try to handwave away why its such a massive dump when it had so many things stacked in its favor that it should have been the first place in the entirety of the wetsern U.s. to rebuild.