This is in part due to the relative complexity of setup. You do not get your grandma performing a one click install of Drupal and then spending $40 on a theme and buying an extension or two from a marketplace.
The focus on earning money with Drupal really seems to be in being a good developer and knowing how to use the thousands of opensource tools that are available to Drupal. You do not see people cranking out apps or modules for drupal and selling them in a marketplace. This really sets it apart from it's php cousins Magento and Wordpress.
You can literally build anything in Drupal, although I'm not sure you should...
In a Drupal custom module you can do or change literally anything, and you can write whatever architecture you like for your individual component. You just have to integrate with the Symfony2 router and dependency injection container. Pretty cool stuff.
This looks interesting indeed.
May I ask what is your preferred way to learn Drupal for a total newcomer, albeit a truly seasoned developer?
It's a paid monthly service, but well worth the educational value you get out of it (for a few months at least).
Good luck!