Money is the reason but if most people would read the manuals and appreciate the manuals then they will provide good manuals. Manufacturers only spend money on things that their customers perceive as value. To many people the manuals have no value and thus to the manufacturers it's not worth it to spend money producing them.
Yes but you are summing up the entirety of modern business here. It's not just about manuals, it's the same principle for absolutely everything.
What I'm not completely convinced about anymore is the recurring narrative about how customers drive the offering of businesses through the perceived value.
While this is part of the equation for sure, the other part is the converse. Businesses drive the perceived value of customers via various marketing tricks, sometimes (and these days, more often than not) using regulations as a pretext (for instance: we'll stop providing paper manuals because it's so bad for the planet - for a while, you'll get the same manuals on CD or USB keys, and then manuals just go out of fashion gradually.)
There's also the related, group effect, where the majority of businesses in a given market make the same kind of decisions because they sound trendy and, rather quickly, customers have no choice anymore (nowhere else to turn to), so a given demand disappears for lack of offer. "Demand and offer" is an intricate feedback loop and certainly not a one-way street only as we still hear way too often .