Part of it is momentum, they date back from when a) everything was on paper, and b) producing and distributing the paper was hard.

However, I suspect much of the staying power is that organizing and executing on a top tier journal, or equivalently replacing it, is real work - and it is real work with no model for getting paid for or otherwise compensated for in an academic career.

There is also some value to being arms-length. A few top universities could gang together to pay for a flight of journals, but there would understandably be concerns about acceptance policies, etc.