There has already been plenty of things written in the past about how to "come up with" plot hooks. We don't need another chapter written about the process of doing it. However, there are plenty of players for whom inventing plothooks is not a strong suit, especially ones specific to the setting for which the book is placed. So having lists of setting specific plothooks to pull from is probably more useful to more people than it might be for you.
I for one know how to come up with ideas... I just don't often feel like the ideas I come up with are as interesting or creative as the ones I get from other people. The same way that I could write all my own adventures for my players, but I know other writers are better at it and come with more compelling characters and stories. So by the same token I wouldn't necessarily want a book that tells me how to write my own adventures when I instead could get one that had a grand adventure already written out.
Some people don't want or need any of the 5E adventure path books and instead just make up their own-- just like you don't want or need charts of plot hooks and can just invent your own. But it's the rest of us who do find them useful that WotC is catering to. And that's true with all their books, they aren't writing them for the advanced or master players, they're writing them for the new, inexperienced, or regular ones.
