What are your chances of becoming the world's foremost expert on a proprietary content management system (CMS) or on proprietary software in general? Unless you're working for the company owning the software and you get access to proprietary documents or high-level meetings within corporate walls, your chances are slim -— you simply won't get access to all the internal information.
Contrast this scenario with Drupal development, or Open Source development in general. As a developer, you have access to Drupal's complete source code. You can read up on all the discussion that led to any design decisions, and you can tap right into the brains of the best Drupal developers in the world.
Why would you want to provide consulting around proprietary software when you know that you will never be able to truly master what you claim to be an expert in? For people with raw ambition, the thought that they can't become the best must be frustrating.
Not so with Open Source software. There is nothing that stops you from becoming the best Drupal developer in the world. The only limitation is your willingness to learn ...
For me that simple and liberating idea makes all the difference.