Although, I guess maybe my argument is moot since your average home PC CPU is going to have at least 4, probably more, cores...
For example, Dwarf Fortress is one of the most complicated games of all time, and the creator wrote about how he kept to just running it on 1 core.
On top of that, you need to test it, which means it's much more time consuming.
I'd be surprised if any complex modern game had multithreaded gameplay code, and even if they do, it's usually some common, but CPU intensive problems, like pathing that gets parallelized.
Additionally, I found that most people don't have the skillset to write correct multithreaded code in any capacity, and even those that do frequently introduce hard to detect bugs, in addition to being a huge mental overhead.
It's even harder to make performance gains using multithreading. Let's say I multithread my code, but the additional complexity, and synchronization makes my code run at 60% the original speed. On 2 threads, I'm running at 120% speed. It would've been much easier to keep my code on 1 thread, and invest the mental energy to make the code 20% faster there.