Why do you think the books were ghostwritten?
First, allow me to say I am not an English professor or something, but I am someone who enjoys reading series'. Piers Anthony's Incarnations of Immortality is my favorite. I guarantee he wrote all 7 of those books; two drafts in pencil, final draft typed.
I enjoyed The Ark well enough, as in I was able to sit and read it in a couple of days or whatever. The second book (Trident's Forge) absolutely did not read like the first. Sure, it had the same characters, but it picked up at a weird spot that the first book didn't allude to (The Ark says specifically that the people on the ship had researched and scouted the planet before hand, and now their are some weird allusion to native American's there), and is written more like a cheap exposition on humanity and how we interact with the environment.
The Ark was more of a technological spy whodunnit conspiracy thriller with a bit of romance and sportsball. They do not FEEL like they are written by the same person. They do not READ like they were written by the same person. This is even more so when you read Starship Repo; a book that should be rife with technology and cool space shit. I just finished that, and there were (I think) two actual "spaceship repos", the rest was the MC being a Mary Sue. Again, it doesn't feel like it was written by the person who wrote The Ark.
Second, ghostwriting is extremely prevalent in the fiction world. A quick check on Fiverr will show many people who will poop you out a novel for about $0.10 a word. This is for a US based author (or company), using an outline that you provide, and only takes a few weeks. Think of it, 80,000 words for $8,000, and suddenly you are an "author" and can change your profile on Twitter. For someone with marketing ability, or the budget to pay for it, you can start a career from there. I'm not saying that $8-10k is chump change, but if I really wanted to be an author I could figure out how to come up with $10k. I was watching a YT streamer the other day speak about how he had been approached to GW for a fairly "prominent" indie author, but declined because he doesn't do it for money. There is also a person I met whose daughter is a GW for a successful Christian author, someone who puts out enough content to make you wonder how he does anything but write.
It does make you think, though, that if you are good at writing why wouldn't you just write and publish your own books? Simple. A mediocre book might make $10k over the course of it's life on Amazon; 8-10 years+. Obviously if it hits that is one thing, but if you have looked recently there is a lot of competition in nearly every genre. If a GW can bang out 80-100k of trite drivel in a month, that's a pretty flash paycheck for someone in their jammies. If you can get several clients, and be the one managing teams of Indians or whatever, then just hit the final polish from there, that could be a massive income. The people paying for their novel to be written don't appear to be very concerned with the final product, rather, they are marketing people just trying to fart out as much content as possible.
Thank you for coming to my TEDtalk.