A big part of game length, without meaning to sound mercenary (just a realist) is money. Yes, I do write for pleasure, but it’s also a profession: releasing a computer game, polishing it, editing it, balancing the stats, that’s all a job. Going through edits and beta testing, that’s a job.
$5000 for a 100,000-word game works out at 5 cents a word. So, realistically, with a game being 200,000-words to even be considered worth someone’s time (again, 200,000 words is the first three Harry Potter books combined)… the writer is being paid 2.5 cents a word. Go up to 500,000 words, and it’s 1 cent a word.
The average rate for a magazine feature etc is about 75 cents a word.
So there’s a pure survival aspect to this. The writers cannot afford to live on the games. They can’t spend as much time as they want creating huge games, which take hours upon hours to write, because it takes time, it takes effort, and the bigger the game, the smaller the pay (and the longer it takes the game to come out, so the longer the turnaround etc.) And while some writers have huge patreon followings etc, the vast majority do not.
The other aspect is that you’re given milestones on when different parts of the game are due. And, as mentioned, when it’s not enough to live on, you have to do it in your spare time - evenings, weekends etc. I don’t think it’s possible for me, given a full-time job and a life, to produce 100,000 words of quality that I’d be willing to share every single month. 3500 words a day, every day, no matter if you’re happy or sad or ill or it’s your birthday or your friend’s in trouble or you just want to chill out and relax. Even touch-typing, that’s at least an hour a day. And that’s before you get to coming up with concepts, descriptions, coding, debugging, editing, proofing…
As I said, there are some amazing authors who have fan backing and the time, patience and work/life balance to deliver that. But there’s a real-world element too if you want to do this as a job, rather than as a past-time.