Most publishers own the rights to the articles in their journals. Anyone who wants to read the articles must pay to access them. Anyone who wants to use the articles in any way must obtain permission from the publisher and is often required to pay an additional fee.
Although many researchers can access the journals they need via their institution and think that access is free, in reality it is not. The institution has often been involved in lengthy negotiations around the price of their site license and reuse of this content is limited.
Paying for access to journals makes sense in the world of print publishing, where providing articles to each reader requires the production of physical copies of articles, but in the online world, with distribution as wide as the internet's reach, it makes much less sense.