Authors and publishers are trying to make money and have the head start on schools and public libraries.

Education systems are typically geographically fragmented including schools, libraries, museums etc. This reduces the scope for scale discounts, but publishers do have deals including physical book discounts and ebook license discounts for bulk usage. After all a library system is just another customer and they are still trying to make money.

Some countries do have a National Library system where certain libraries can demand a few copies of any published books for free. This is the exception, not the rule, though.