You have to pay them, and most of them are not very good at writing. Even with a big budget, you get a limited number of good articles per day.
If you can make writing fake news 100x cheaper, and then just throw everything at social networks and let people sort out the most viral stuff, that can change the game.
Also, computers can be faster. If something new happens today and hundred articles are written about it, a computer can quickly process them and generate hundred more articles on the same topic, than a group of humans would. (Many humans can do the writing in parallel, but each of them needs to read individually the things they want to react to.)