Simply put, Hiram Gummer kept on living, and due to being really old has had his memory fade so that he doesn't remember the Graboids (or Dirt Dragons, whatever), only that there's some terrible danger to be paranoid of. Over the decades he's likely projected that fear onto the Communists and even the U.S. Government, thinking those were the real threats.
- Supporting fact: Burt is the only character in the whole series to survive being swallowed by a Graboid.
- Until the fifth movie, that is.
We know that shriekers are able to breed parthenogenetically, because we've seen it happen. It's likely that Graboids are either asexual or self-fertilizing, as every Graboid (except El Blanco) that lives long enough gives "birth" to shriekers, even Graboids which hatched out alone. But it's almost unheard-of for complex multicellular life forms to forego sexual recombination completely — there's too much evolutionary advantage to be had in creating varied offspring — so it's plausible that at least one stage of the Graboid life cycle would reproduce sexually.
In the third movie, the ass-blaster that killed Miguel was examined after death, and found to be carrying eggs. These eggs must've been produced very quickly after it metamorphosed from its shrieker form, as the entire film takes place over only a few days; however, this isn't out of line with how rapidly the Graboid species appears to grow at all stages of life (fast metabolism). Presumably, it — rather, she — would've flown off to lay them as soon as she'd eaten enough to sustain her for her journey. The other ass-blasters' ravenous attacks may have been spurred on by that same drive to migrate and disperse eggs.
And yet, the one ass-blaster to survive the films, the captive Messerschmitt, opts to return to Perfection upon escaping, rather than flying off to lay eggs after consuming the thieves who stole it in Vegas. No mention is made, in "Blast From The Past", of the risk that it might lay eggs; once identified as Sigmund & Ray's animal, it's referred to as "he" throughout. What's more, all other Graboids or shriekers that appear in the TV series — even the ones purchased for research — are derived from wild-caught specimens, not Messerschmitt. Were ass-blasters capable of parthenogenesis, you'd think that Sigmund & Ray would have an ample supply of eggs to sell to interested researchers ... and that we'd see larval dirt-dragon models in Jodie's store, as well as other life stages, since these would've been observed when Messerschmitt's eggs hatched.
Conclusion? Messerschmitt really was rightly referred to as "he", hence had never laid eggs, and had flown all the way to Perfection in hope of finding other ass-blasters — i.e. fertile females — still there. His picky eating habits were as much a result of his having sex on his mind, rather than food, as of having been spoiled by Sigmund & Ray. The fact that he's male, and doesn't have to fly long distances to seek a place to lay eggs, may even explain why he fell into a "food coma" in the first place: he didn't have the use for all those excess calories that a female ass-blaster would, so took a nap rather than flew off to find a nesting site. Nancy and Mindy are only alive today because it was him, not one of the females, that spotted their body heat on the rooftop.
Cheer up, Burt. You really did need to blow up your bunker: eating MREs would've only put the male ass-blasters to sleep, whereas the females would've scattered across the state and spread the Graboid menace beyond Perfection Valley, with no granite barriers to confine them.
- Ever heard of the Whiptail Lizards? All of them are females, and while they simulate mating they lay eggs, and the ensuing offspring is an exact copy of its mother. It's possible for Graboids be either genderless or at least an all-female species that reproduces via parthenogenesis.
- Yes, I have. I never said it was impossible for them to be fully asexual, only that it would mean forfeiting a major evolutionary advantage to become so.
As for how the Graboids got into Perfection Valley is anyone's guess *Cough Arceus Cough* But all I'm saying is: Wouldn't it be fun if somewhere in southern Orre, there were a few fossil pokémon with a certain lifecycle?
As a final piece of evidence, I'll let you chew on this: El Blanco is a Shiny graboid.