I can think of a couple of options, some of which had already been stated.
It could, of course, be for the money and the prestige. Sure, you could get someone to cast Miracle or Wish five times for the same effect and less cost, but that requires you to have the relevant spellcaster right there, with you. And it's not like it would be a private matter. I can easily see certain kinds of people wanting to pay the extra cost for the increased convenience and privacy, especially people who might not get on with clerics. There's also the prestige involved in not only being able to make these, but being able to afford one. Some people will pay truly ridiculous amounts of money for certain brands or designers. I see no reason magical goods would be different. And unlike the 'Miracle 5 Times' thing, you could actually point at a book and say 'See? That's a insert name original~" So it's not like a competent mage would have a problem finding a market for these things, it would just be a very select market. And if they're in that strata of society, they could have a backer paying for the material costs.
Similar to the above motive, it's possible that someone has enslaved a bunch of casters and is forcing them to create these and other artifacts for their use and profit. In that case, the creators of the items are unable to enjoy the fruits of their labors. In that case, the creation time and all non-material expenses become irrelevant, because what does the taskmaster care? Material costs, such as diamonds, are almost certainly provided through other slaves or agents.
It could be that these books are meant for someone yet to come, from the before-mentioned prophesied grandchildren, to heirs of a dying mage, to The Chosen One(s), because there is always a Chosen One. Whether or not the Chosen One gets his hands on the book first is another matter entirely.
It could be that they're divine boons... of one sort or another. Maybe some god laced the world with these to help his followers. Maybe some evil entity created them to sow havoc, not unlike the Golden Apple of Discord. These things are rare, they're valuable, and they only work once. Blood will be shed over them, either now or in the past. If you go with this option, then I would say that mortal casters can't or won't create them, and just ignore the crafting rules for them.
It could be that they're bait to lure adventurers into some trap. Maybe they're just located in a trapped library or with some terrible guardian. Maybe the trap is in the book itself: they get their stat bonus, but become the target of some spell or curse. Maybe they're a way to mark people of interest (because anyone who could get to it is bound to be of interest), for later use. Some sort of identifier in the magic, or a geas spell that hasn't gone off yet but will trigger when the time is right... could be anything. No one would create these things without some material benefit, and since they're not using the bonuses themselves, they've got to be getting something else of equal or greater value. Like, say, a healthy and lucky adventurer.
This could be slightly off the rails, and I don't know if it violates the rules laid out in the item description, but it's possible that the tomes could be read by multiple people in a short span. (Case in point, there are some books in World of Warcraft that give you an achievement if you read. Once the first person reads them, a timer starts. Anyone else can read the book in that time span, but the book vanishes when time is up.) If that's not actually against the rules and you choose to use it, then these tomes are likely to be commissioned by guilds or academies for some of their most promising members, and access will be strictly regulated. They'd probably only get one once or twice a year at most, and there's probably going to be some big to-do coinciding with it. Graduation, a tournament, a festival, what have you, where they publicly announce who gets to read the book. In addition to a time limit, it's also possible there is a beneficiary limit. Like, the book lasts six days once someone starts it, and only six people can read it. Whatever works.
Also, having not read the actual item description... is it single use per character or single use ever? If it's the former, then there's no problem at all.
Edit: I suppose it could also be some philanthropist with ENTIRELY too much spare time, but really, where's the fun in that?