I admit that I have only dabbled at the surface of Mac and Windows GUI programming, so what I write below can be taken as an ill-informed rant if you so choose. With that caveat…
It has always seemed to me that some of the GUI differences between Windows and Mac SketchUp come down to lack of will to spend the resources to unify them (or historically perhaps sheer lack of resources).
I’m not talking about “the Mac look and feel” vs “the Windows look and feel”, for example how Mac palettes float atop each other rather than snapping together; it’s a question of available functions. I get it that there may be a vendor library on one platform that has no direct equivalent on the other. But that’s a poor excuse to provide a significant capability on one platform and then leave it off the other completely or provide a much different and less capable alternative!
The ability to make custom toolbars is a good example. Windows SketchUp provides a very flexible way to do so - the dialog @Box showed is part of that system. A user can create multiple custom toolbars, each with an arbitrary suite of tools.
On Mac SketchUp, there is a single toolbar stripe across the top of the SketchUp window (not talking about the app menu at the top of the screen - that is indeed a Mac look and feel thing). A user can drag individual command buttons on and off that toolbar stripe. But it is the one and only, is fixed size, and can’t be repositioned anywhere else on the window. This is not standard Mac look and feel. I don’t know of another Mac app that works that way, though some support custom shortcut menus and tabs up there. So, this doesn’t seem like an issue of “the Mac way”. Rather, it seems like they didn’t want to spend the development time to come up with a unified capability across the two platforms.