You may have a warped view of motivational books; or what books truly fit the definition you described might be vanishingly small compared to the total number of books that are called "motivational".
A better name for them would be cognitive-behavioral books, because the majority of them focus on something you can do differently in order to increase your success. The goal may be in some cases to increase happiness, and the prescription may in some cases be cognitive, i.e. "be thankful for one new thing each day and be happier!" But often the goals and strategies fall outside of these categories.
While many of the information has already been thought of and written down and is now only being repeated, isn't that always true of common sense advice? And what other kind of advice is worthwhile besides the common sense kind? If you seek out the alternative to common sense, you will only end up with increasingly obscurer and more complex drivel. What's more, the presentation and organization of the old material may make it more novel or more accessible. Perhaps even with varying success for different people depending on personality and aptitudes.
Moreover, in enacting change within oneself, especially mental change, there is much to be said for repetition, as the subconscious mind works off of what it was fed most recently. Therefore, a habit of reading motivational books may incur greater benefit than simply hearing the ideas once and moving on, even if the information between them overlaps heavily. There is enough novelty in the presentation to keep the stimulus novel, and the repetition strengthens the presence of those beneficial ideas within the mind.
My main gripe is that most advice from these books are incomplete, broken, shallow representations of the wisdom that's already been passed down throughout human history. The incomplete nature of these bits of advice, in my experience, end up leading people down a negative path, because the scope of the "wisdom" is so narrow.
I would posit that the wisest among us didn't get that way by reading motivational books nor cognative-behavioral books. They got that way by studying history and philosophy. Not the vestiges of wisdom present in contemporary literature.
I'm not taking the position you shouldn't study philosophy, classical works or religious texts obviously. But you might be surprised what the wisest among us read. (I am not including myself in that category, though I do read motivational books. Other side of the Venn Diagram.)
Not to mention the whole motivational genre has its own canon of classics. Think Dale Carnegie, Og Mandino, Zig Ziglar. I think you would be hard pressed to find a wise man who hasn't benefitted from one such, though I don't know a lot of them personally.
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