It's not people who have big ideas but are gaming the journals who are doing this, come on. You can write a book and get it published. You can co-author some and solo-author on some papers and or a book. It's allowed. No one has a problem with people who mix it up. The people who are defensive about this- ask yourself why you even want to do this when you have nothing to say. For summers off? For "status"?
It's individually rational, yet the outcome - a field full of RAs who get tenure and become TERRIBLE reviewers because they know nothing about anything- is pathological.
In a world where top journals are guarded by gatekeepers, reviewers are random and capricious, and the field revolves around networking, co-authoring is a rational decision to reduce risk.
If you want to change it, improve review times and stop gatekeeping. Journal space is now a scarce good. Nobody wants to end up on the street at the age of 40 because you rejected their 'big idea'