Because humanist.

To illustrate, take two characters, one from Adams, the other Pratchett. Prak is a minor character from Life, The Universe, And Everything—the third Hitchhiker’s book, and Adams’ best work—who exemplifies what life in an Adams book is like. Accidentally imbued with omniscience due to a clumsy accident—he was injected with far too much truth serum, in one of those inspired bits of literalism the author was so fond of—Prak eventually dies when he encounters Arthur Dent and can’t stop himself from laughing so hard, and so long, that it completely wrecks his body. Prak is the perfect Adams character: His entire life is an absurd, nasty, hilarious joke, and it perfectly captures the borderline nihilistic comedy of the Hitchhiker’s universe

Compare that to Reg Shoe, a recurring character from Pratchett’s work. Reg is initially presented as a joke, too, an “undead activist” who campaigns for equal rights for his fellow zombies and vampires, despite the fact that most of them are only involved in the movement to humor him. (His denial-filled focus on improving their lives almost puts him in line at times with the classic Pratchett villain, who ignores being good to people to focus on The Good of The People.) He’s a throwaway character, a one-off joke from 1991’s Reaper Man. Except, by then, Pratchett wasn’t throwing characters away anymore, if he could find somewhere useful to fit them in. And so Reg continued to appear, primarily as a member of the Ankh-Morpork City Watch, the framework on which so many of Pratchett’s best stories hang. When the time travel adventure Night Watchfinally brings the reader to the moment that transformed Reg into a zombie in the first place, Pratchett has managed to imbue this joke character with a nobility that makes the moment one of the most emotionally powerful in the entire series, all without tossing away the element of the character—his delusional idealism—that made him so funny in the first place.

Of course, “emotional power” was never Adams’ goal. (The insistence on inserting those kind of moments into a world with no room for them was just one of the many sins committed by And Another Thing..., Eoin Colfer’s well-meaning-but-awful Hitchhiker’s sequel, written after Adams’ death.) As a devotee of P.G. Wodehouse, Adams was a master of farce, a believer in finding the most perfectly crafted line in any given situation, the most expertly set up joke. It’s that distinction, which widened as Pratchett grew into his strengths as a storyteller, that most clearly puts the lie to the idea that Sir Terry was “the fantasy Douglas Adams.” In truth, his work has far more in common with that of some of fiction’s great humanists, like G.K. Chesterton and Kurt Vonnegut.

The Vonnegut parallels are especially apparent; neither man was a scientist, but both had great respect for the field, and both worked in jobs adjacent to it. (Vonnegut as a public relations worker for General Electric, Pratchett as a press officer for the U.K.’s Central Electricity Generating Board.) Both struggled with the restrictions of being labeled a genre writer, although Pratchett’s comments on the subject were a lot gentler than Vonnegut’s description of science fiction as a file drawer that “serious critics regularly mistake... for a urinal.”