When you have a system.object reference you can click on the null object in the in the inspector to choose what you want to assign to it. However, if you type in the search bar System.bool or System.string, you are unable to assign it, to their right it says "No default constructor found" and clicking does nothing.
However, if you assign a boolean or string through code, it shows up in the inspector and you can edit it and everything. I've attached an example script. Assign it to an empty gameobject. When you hit play, you'll see that the booleans get assigned and you're able to edit them in the inspector. But manually clicking the object and selecting bool, string, etc. doesn't work (but it does work for regular classes).