Radroaches appear in several spots in New Vegas. I believe there’s actually one just outside of Novac that fights a Gecko as a pre-determined little encounter. They also appear throughout Dead Money in little nooks and crannies and in buildings the fog and Ghost People have not worn down and broken into yet. There’s one particular spot where about 3 or 4 of them ambush you on one of Domino’s supply caches in a back alley. Very cinematic, very actionable.
But thats New Vegas, Im talking about the general California area that we saw in F1 and 2.
You can argue semantics and say that they simply spread over to that area or that they always existed and simply never made a major appearance in that particular area...
But fuck it, the point of my argument is the Bethesda-fication of the Interplay lore, including its locations having new creatures that have never been there, like Gulpers. Meanwhile Geekos are nowhere to be seen. We all know the answer out of verse because there is no in universe explanation beyond "they just are" or "who cares"
I hate the show for this a lot. I think they might backtrack on calling it canon in the future.
Bethesda always had an iffy relationship with canon consistency, even with their original franchise The Elder Scrolls. Its easily one of the most frustrating things about being a Bethesda fan because you cant get too attached to a version of events because Bethesda will easily retcon it if they deem it necessary for their current plot to work, if it makes sense or not is rather optional.
Every new Fallout game of theirs retcons something or contradicts lore established either by Interplay, Obsidian or fuck, even themselves ( I already ranted here how you needing to shoot ghouls in the head to kill them contradicts one of the best F3 missions right down to its very title). Shockingly I think that comes close to not doing this is F3, sure, you can say that some of its original lore is flawed but at least none of it seems to actively contradict the previous entries (tho Im sure you can find something, I just cant recall one right now) and it seems Bethesda decided that "its a series with green large atomic men and radioactive zombies, who cares for internal consistency" and decided to do whatever they wanted, tho dont worry, Emil gives "a lot of damns" about the lore (mf cant even resist throwing a tweet retcon in the form of Nate The Rake, which he was so unironically shocked that people turned it into a mocking meme that he "unretcon" it).
What Im saying is that as long Howard and Emil is around, live in the comfort that your fanfictions (probably written better) are as canon as the canon itself.