First off, that is incorrect. You underestimate the number of VB, C#, and other apps, not to mention OSX based apps. Most business apps are either in Java on the backend or if they aren't web based they are in proprietary languages and frameworks that likely use a mix of C and C++ at the core. They are mostly shit nowadays though which is why everyone is re-writing them in the Cloud. They're unmaintainable.
But yes there are a lot of high visibility popular desktop apps in C++ and yes, it is just inertia. When were all of those desktop apps you're thinking of first built (or came of age)? The early 90s! Microsoft Office, the Adobe suite, Visio (bought out by Microsoft), desktop databases, etc.
That was the age of the desktop and briefly, C++ was the best choice. I'll grant you a few very small niches still exist and they are in the desktop domain e.g. web browsers and very complex image editors. But how many people are writing browsers? And simpler image editors are better off being mostly in Objective-C or C# (minus some core libraries which are best off in C).