Because it’s just a video game boss battle.
What is the tarrasque, anyway? An oddball concatenation of miscellaneous special abilities fulfilling little purpose save the gamist impulse to kill the biggest thing in the setting.
It takes its name from the tarasque of Provence, a lion-headed creature combining the features of a number of beasts and said to lay waste to the countryside. But there the similarities end; the Provencal tarasque met its end not in epic combat, but rather was tamed by the prayers of a saint. Outside Provence, the legend never seems to have captured the public imagination, but rather seems an allegory of Christian conversion and the backlash faced by newcomers to the flock.
Monsters become frightening, awful, or awe-full when something about them touches on something in us deeper than large numbers in a bestiary. Some monsters–e.g. ghosts, werewolves–embody common human fears, here maybe the fear of unfulfilled purpose, of losing control of one’s self. Others represent sin or taboo, embodiments of those urges we would extirpate from ourselves. Vampires, the sin of lust, unfulfillable and ultimately damning; wendigo, the taboo of cannibalism, beginning tragically and ending worse.
But chimeric monsters just feel like lazy mythmaking. “It’s a ferocious monster!” “How do we know?” “Uh, because it’s got the body parts of various other ferocious animals all mixed together?” When chimeric monsters work, when they resonate in the imagination, we stop thinking of them as chimeric, and just think of them on their own terms. A centaur is a chimeric monster, a horse with the torso of a man. Morphologically, an angel is just a person with birds’ wings. A pegasus, a sphinx, these stand on their own. They work. If our tarrasque were closer to the Provencal tarasque, maybe it would work better.
But it didn’t. It just never caught the imagination well enough. Have your players ever fought a tarrasque? If so, was it a roleplaying experience, rich and evocative? Or was it just the biggest boss monster in the book, and so the logical thing to fight at the end of the campaign?
Quick, imagine a tarrasque. What did you come up with? It’s… big? With a shell? Maybe a shiny shell? It probably bites things? We might all disagree about what a zombie looks like, but we can each imagine one, and do so effortlessly. If your players can’t instantly and satisfyingly imagine what your monster looks like and does to you, you’re unlikely to have a satisfying scene. How much more the pity is it when that’s the Big Bad of your whole campaign.