It takes a lot of money and resources to be self-employed in this industry. Along with a whole lot of contacts, and a good reputation for working well with others and providing quality work to your clients.

Being self-employed means you're always working. You don't have the luxury of taking a vacation, or even a day or two off when you want to. At least not in the beginning. Not until you're well established and have a team of artists working for you to meet your deadlines and a reputation that is gaining you future contracts. Because even when you're not directly working on a project, you're spending your time marketing yourself, acquiring more contracts.

And you still have bosses even when you're self-employed. Every client is your boss, and when you don't meet their expectations, they find someone else who will, just like any employer. Your goal is to sell your talents and expertise to them - they're not selling anything to you and there's a thousand other artists and studios out there competing against you for that same client. It's your job to convince that client that you're the best man for the job, at the best rate with the fastest turn-around and the highest quality. They don't want to hear you can't do this or you don't use that software. They want results and they don't care what your excuses are. And when you fail at one job they hire you for, your chances of them hiring you again for your services drop significantly, as do your chances of acquiring another client, because word can spread pretty fast especially on the internet. One sub-par project you do for someone is ten times more likely to damage your reputation in the eyes of other potential clients than 10 perfect jobs will do to increase your reputation. That's just how it is, in any industry. People remember when they get crappy service and are quick to tell everyone they know about it. Those same people aren't so quick to talk about the great service you provided unless their friends and colleagues specifically ask them about you. 

And even when you're running your own studio, you still have to use software that you may not agree with, to meet those clients expectations. If you're producing a complete film or video game or whatever, a final finished product for a client, then no - the client isn't going to care whether you used gimp or photoshop, or blender or maya. But a lot of the time - far more often than not - you're not producing a finished product for that client - you're only producing one or two aspects of it - whichever area of it that your studio specializes in - such as an animation sequence, or a group of models. That client will then take those files and hand them off to the next guy who will continue with it, and the next guy and the next, until the project is complete. Clients often build a team of multiple artists or studios that when combined will produce the best results the fastest, and at the lowest rate possible for their budget. So if you're using software that the rest of that client's team doesn't use - because it's not an industry standard - then your chances of that client hiring you fall to practically zero. 

As for gimp being "the world's most popular 2D editing software" - maybe the gimp's maker's and it's loyal users believe that, but if that were truly the case, we'd all be reading and hearing about it more on various sites. The only time I ever hear about gimp is when I go to gimp's website. When I take files to be printed they want them in psd or some other adobe format, because that's what the majority of printers use.