It’s because you bought the the right to read a Kindle book, not to own the the digital file. It’s not in the interest of either the publishers of digital books, Amazon or authors to allow people to make copies of copyrighted material the company or author owns. The person who copied something from the digital file could easily share it, or a whole book, with many others. It’s why publishers, Amazon, Apple and other eBook retailers limit the number of people you can share eBooks with. Some don’t allow sharing of certain books at all. If you buy a PDF copy of a book, it’s typical for it to be an uneditable or uncopyable version. But when you buy a book that’s made of paper and cardboard, you own the physical materials and can share or sell it however you want. The fact that it’s extremely expensive to copy a book printed on paper is a deterrent enough for illegal sales.
If the info you want to copy is in the Kindle app on an iPad, Mac or iPhone, a screen shot of the page might work, but I’m guessing about this.