The legal standard is so high very few of the type of case you describe result in a win. That is why they are hyped so much when they do win. Salacious accusations of the he said/she said variety are not winnable but can be very damaging. As with all the left has thrown at Trump, the proof bar is very high for independent and Republican voters because the left has claimed "bombshells" like the boy cried wolf...
I'd imagine that when a book is written, full of libel, then that high standard is probably easier to satisfy.