Because journals contain up to date information and findings whereas books usually need too much time to be published. Their contents whether have abstract knowledge or old information
Original work is in journals while books are usually secondary sources. Even when books are used the researchers are expected to trace the source to original publications.
Published articles are easy to obtain and usually contain applied information that supports what the researcher's needs besides due to the high prices of books.
Initially, you must start with the verb ARE in the 2nd question. Further, regarding your question, it happens for various reasons; easily to approach, free access, updated as well as the world is becoming digital and highly affected by technology and research is not an exception. Online studies usually end with conclusions which might be extremely beneficial for new studies.