Welcome to capitalism, where profits of a few are more important that scientific improvement.

Publishers don't give a s%it if the articles aren't available to everyone, they want to make profit from subscriptions. Authors are kinda forced to publish their papers on the mainstream vehicles, so they don't have a choice.

It has nothing to do with research costs like some people here are saying. Neither the authors and universities get a penny of the subscription money.
However, if you email any author asking for .pdf of a paper/article that he wrote, he'll gladly send you for free, also there are people fighting this, like Alexandra Elbakyan, with scihub (If you don't know her, look it up, here's the transcript of her presentation "Why Science is Better with Communism? The Case of Sci-Hub." https://openaccess.unt.edu/symposium/2016/info/transcript-and-translation-sci-hub-presentation).