Because historically (and for the most part today as well), it costs money.

Sure, today stuff like ChatGPT and the somewhat older Google Translate exists, but that doesn't solve the cost issue. (And I'm skirting on the huge elephant in the room called quality for a bit of brevity).

There's a huge chance someone paid a good chunk of money for all the books you find dirt-cheap at a flea market, check out at a library or happen to find in your own house.

Printing physical books is expensive. Publishers also want a margin, and a lot of authors want royalties.

In the end even if the publisher and author are both good souls demanding nothing, someone needs to foot the cost of printing. But before that, you'd need to go through non-trivial talks with most authors' publishers and/or authors themselves.

Then you need to arange for translation, typesetting and printing if you're not doing it yourself. That takes both time and money.

And if you were to do all that yourself, it'd be a huge time investment, with a potential lawsuit if you don't do those damn talks. So most just don't bother.

Businesses are incredibly inefficient, even though some are "successful" and have a lot of cash to burn. They need to pay workers, bills, buy and fix equipment, and of course, a cut needs to go to the top people. Usually the "golden" 80-20 rule applies to almost everything: 20% of books make 80% of money, 20% of employees make 80% of money, and a different 20% of people do 80% of the work, etc. And of course, in this world, it's all about the money.

A translation is usually initiated by a publisher that has a manager who wants to get his section's metrics up to go cry to his own manager about how good he is to get a raise or not get fired. This is a daily grind. Sometimes (but quite rarely), that leads the manager to the decision of publishing a new book. Usually such actions are guided by things like bestseller lists, reviews and personal biases of the manager and the company as a whole. Sometimes the publisher hires an agency to try to approximate the demand for such a book (even more money spent). Then they do the talks. This also costs money, and the result is also a cost of money (the royalties to be paid). Then comes translation, then printing, then distribution to bookstores, and finally advertising.

These are just the steps that come to mind. All cost money, and all the books you see for sale in a bookstore went through all of these steps. For a library, not as much (but still the vast majority) did.

Sure, not every situation is the same, so there are companies that specialize in providing translations of well-known works or companies whose manager at one point said they need to publish 25 translations yearly (instead of one individual one), so they kind of "flood" the market.

But sometimes it's just the whim of a newspaper whose management thought printing classic works of shorter length and bundling them with their newspaper would drive up newspaper sales.

It's incredible how each document (edition of a book or otherwise) has multiple stories (of the author, publisher, translator, seller, advertiser, buyer, worker in logistics/delivery driver,...) that shaped the life of it. Some lasted a few hours, and some took hundereds of man-hours. All of this somehow translates to money.

That's the long answer.

The short one is: 80% the economy and 20% human laziness.