Why aren't the books longer?
If I could still release 2 books a year and have them be 20k-30k words longer than they are now, but make no extra profit, I’d do it in a heartbeat. (I wouldn’t literally double the size of the books in the middle of a series, that would be jarring, but I could imagine a substantial increase of ~25%.)
But it’s like you said: if I take that whole year to write one double-sized book and release it, I’ll have fewer people reading it. So I’ve done two books’ worth of work for less than one regular book’s worth of visibility and engagement.
I can’t maintain momentum at that pace, so I’ll slowly lose visibility until I vanish in the sea of Kindle Unlimited.
Also, I have people emailing me angrily every time about having to wait six months. I can’t even imagine the response if I said I was adjusting my pace to one book per year.
There are a lot of reasons, but they mostly relate back to momentum.
I’m reliant on Amazon for all of my marketing, which includes letting anyone who isn’t subscribed to my social media accounts know that the book is released. Most of my readers are not following me for releases, they simply notice that a new Cradle book is out and go buy it.
Amazon is geared toward a structure of pushing the leaders, i.e. the more you sell, the more visibility you get on the site, and the more you sell. As long as you keep pushing that wheel of releases, you stay highly ranked on the site.
And I am dependent on that rank to reach most of my readers. The MOST efficient release schedule would be a book every 1-3 months, but they would take a...significant drop...in quality if I tried to write them in that amount of time.
I’ve stuck to a schedule of 2 books per year, which I’ve been able to do because I’m already established and have a large fan base, but I do see a huge dip in reader engagement and awareness between those two releases. I’ve been told by many other authors that this schedule is not frequent enough or maintainable...except that I’ve been able to do it.
High-profile published authors can do otherwise because their publishers handle the job of getting the word out to readers, but since I’m dependent on Amazon, the name of the game is momentum.
So...given that I have a finite time frame within which to write and produce a book, I have a finite word count, and trial and error has taught me that the sweet spot for that word count is about 90-100k words.
There are many other reasons, such as consistency within a series, but this is the main one. If I only release one book a year, people forget about me, sales and visibility drop, more people forget about me, and then instead of topping the Amazon charts I’m no longer able to write full-time. And then the books take even longer.
That’s the partial answer. TL;DR - It’s about how Amazon works.
However, this answer is complex and always evolving. Any part of it could change as my situation and Amazon’s policies change, which they both often do.