Their value is very similar to that offered by Ivy League schools - prestige. The journals have the reputation for only publishing the highest quality, high impact papers. They act a filter.

Getting a paper in Nature will help you get tenure or win you that grant you need. Publishing in a no name journal is barely better than not publishing at all.

Because these journals are so prestigious, they can charge a lot for access. Every top school will buy access because it would be embarrassing if they didn't.

There is nothing stopping researchers from publishing in their favorite open access journal. In fact, they'd probably have a better chance of getting it published. However, they don't want to - those journals aren't prestigious enough. And without enough prestigious papers, they'll never make tenure.