1. You need enough paper to create an object with a noticeable mass that takes time to work all the way through. Too small or short and it doesn't feel worth it. Make it short enough and people could read it in the book store.
2. People are bad at applying a crystalized abstraction in day-to-day life. They are better at learning narratives and fitting the current situation to the closest learned narrative, and then acting out the part of the protagonist. Instead of explaining a statistic or explicit rule of thumb, it would be more effective to give a bunch of examples where someone successfully applies the rule and is rewarded. Those examples can take up many pages.
Despite constant complaints about the padding in many non-fiction books (not just business) there's clearly a silent majority who feel like "If I'm going to bother, ugh, opening a book then I want it to be as thick as possible".
For that matter, you see a similar dynamic on the fiction side too with novellas and short stories bring far distant in popularity to novels. (Even though those same people have zero issues watching 22 minute TV episodes.)
Then there was the delightful Bluffer's Guides series.
I like shorter novels and short stories myself, especially in SF where ideas matter.