D&D Beyond is a different company that Wizards. They need to pay Wizards a licensing fee for the content as well as somehow pay for all of the effort of digitizing the books and server costs, among other things.

This gets asked very frequently (at least 3 times a week) and has been answered very thoroughly in other threads, FAQs, articles, and YouTube videos.

Some highlights include:

• DDB has to pay WotC for each person they let access a book.
• Putting codes in books will require collaboration with WotC.
• Codes can only be put in sealed products, otherwise someone can steal the code without buying the book (books are not currently sealed).
• Books with codes in them would need to sell for more (around 5-10$).