In the same fashion that a good screenwriter is not necessarily going to be a good novelist, a good novelist is not necessarily going to be a good game writer - and vice versa. Ignoring the medium differences for a second, the environment is utterly different and requires different skillsets.

An author works mostly alone. They have ownership over their story, intellectually if not necessarily legally. They work with some editors, but those editors are people who are there to make the author's story better and more polished: it's still the author's story.

Games are an entirely different beast. You've got huge teams, for one. The writing is a small piece of a larger product, as opposed to THE whole product. You very rarely have a game that is driven by a single individual's creative vision, and when that does happen (Kojima, Jaffe), it's usually by a game designer or generaly creative director of some sort, never a writer. There's a huge amount of collaboration, compromise, organization, and prioritization skills that a game writer needs that a book author never needs to deal with, at least not as often or on such a scale.

Second, the medium is vastly different. How would you go about writing a game script? Think about it for a second. The knee-jerk reaction most people have to this question is that they'd write it like a film script. But unless the game is incredibly linear, that model breaks down. And the right answer? There is none, because depending on the game, the genre, how linear it is, whether the dialog branches, how many side quests it has, what sort of character interactions the game supports, and a host of other factors, the format of your script is going to be different. I've known games where the script can be broken down into a ton of modular pieces, which are organized into some sort of folder hierarchy. Those pieces themselves might be written like film scripts, but they might be written in excel spreadsheets with a bunch of meta info attached to small pieces of script, like what character says the line under what conditions, what might trigger it, what file it is stored in. In other cases, referring to "a game script" is erroneous: there isn't always a single collection of writing that can be pointed at and labeled a script, and depending on what the purpose of the writing is it could end up in a million different places and organized in a number of different ways. And, of coruse, written by 20 different people. I could go on, since the nature of the game industry calls for almost as many solutions as there are games, but you get the point.

And then there is the writing itself. There are fundamental differences between what sort of writing works in a game vs the writing that works in a book. I'm realizing that I've spent too much time writing this so I've gotta be brief, but game writing is going to be different in areas like brevity, how compact the writing is, how much exposition is acceptable, what sort of situations you can expect a player to read th text vs ignoring it, and how you're going to impart important feedback and direction into the writing. And finally, any game with good writing needs to make that writing an important part of the game itself: it needs to be woven into every aspect of world building, character animations, and music, and it needs to support the flow of the gameplay itself, which is where it so often breaks down.

In short: novelists don't know jack shit about writing for games, and just because they can write a good book doesn't mean they can write for a game. The skillsets are very different, the writing is very different, the goals of the writing is very different. And, although I know you weren't really making this argument, the attitude that "oh, games have bad writing, but it would get better if only they brought in 'real' authors" (which also implies that all game writers are impotent hacks!) is an ignorant one held by people who don't understand what they're talking about.