While I agree with you about the observation that many non-fiction books seem unnecessarily padded with filler, I think it’s because otherwise the book looks physically inferior and/or would be expected to cost less. And they’ve decided that this is the page count that optimises sales, instead of optimising reader satisfaction.

If someone’s at the book shop and see a bunch of one-inch thick books and a small 20 page pamphlet, they’d expect the pamphlet to sell for a fraction of the price. Or be tempted for the thicker book because “the author clearly did more work.”