ALL SPOILERS UNMARKED. YOU HAVE BEEN WARNED.
That's why Lizbert was able to make them into the beast; they remember the form they each once took, and they were perfectly willing to return to that form to help a member of their former race, and take a stand against the species that took their lives.
- Adding to this, Sprout and the Kweeble duo also have memories of being Grumpuses, which is why they are willing to help you in the final battle.
Their biology is distinct from any plant's or animal's, and they sprout from Snaktooth Island fully grown.
- Their main purpose is to assimilate Snaktooth's prey, as seen by grown ones acting on a hive mind. They don't pull a siege on Snaktooth until the island hungers enough, and they could have behaved as an impromptu immune system in the Undersnax if Lizbert didn't hold them off.
- Furthermore, the fact that they turn into harmless gunge when cooked or cut implies that cutting them open spills out cytoplasm, and that excessively high temperatures swiftly denature the "cell membranes" of non-flaming Bugsnax.
Continuing from right above and the idea of it getting closer to Grumpus civilization at all, it could collapse for good if it breaking apart proves to be fruitless.
Those who think of Bugsnax as something that can be used "to fill the hole inside their soul" become much more vulnerable to their effects on the mind. A comment on the video for the Bugsnax OST "Escape from Snakburg" states that assimilated Grumpuses permanently unravel because they find such to be the only way to be accepted, loved, and/or confident, even though it actually isn't one.
But due to the expedition's fate resting on them, Lizbert and Filbo inevitably cling on: Lizbert turns the brunt of the snax' abilities against them, and Filbo is the only one you defend at the end of the game who cannot eat Bugsnax that reach him (regardless of quest completion).
A secret organization connected to the Bugsnax is real; unbeknownst to anyone else, let alone one blinded by vengeance and paranoia, they seek to improve on the nature of the critters instead of exacerbating their current one...