How do I stop Blender 2.8 from quitting when I close a project? I was working on a throw away project (a tutorial) and decided I wanted to try a different one. I closed the project, said I didn't want to save it, and it closed, but so did Blender. A bunch of macOS utilities do this, but IMHO it's a terrible choice for a major application.

How can I make that stop? I can't find anything in the preferences.
You cannot stop Blender from quitting when you close a project
I'm afraid, it does not work the way you wish it to work. Blender cannot be opened without some file loaded and it can open only one file at a time. If the file is closed, Blender is closed as well. Even when you start Blender for the first time it loads a file - that is the default scene. You can open multiple instances of Blender with different files. Then if you close one, all the other open instances will remain open. This is just how it works.
You can have multiple scenes in a file and they can have completely different settings - they can even use different render engines for example, only one scene in an open file can be active at a time, but you can switch between them without closing the file or the instance of Blender.