Because opportunities don't fall from the sky for most people. Their side projects suddenly don't become twice as lucrative as their main job, and their job doesn't put them on a path to acquire skills that are worth 2x in the market. You'd have to be incredibly fortunate for this to happen to you.
Nonsense. I changed jobs 4 years ago, and while I haven't quite doubled my earnings in those 4 years, I'm over 50% higher than where I was before, in a job I enjoy more which is closer to home, however to get here, I worked my **** off for a couple of years prior to develop my VB.NET & C# skills in my "spare" time (what little there was trying to also juggle working full time and a young child), and then applied for an internal junior position which involved being seconded to a customer's site 40 miles/~1.5hrs away (compared to the 15 mins on the bike to the company's office). It then took me several weeks of applying, and 4-5 interviews before I got the current position (which initially involved a pay cut during my probation period).
Tell me again about those opportunities falling from the sky?
Surely if most people aks themselves the question "can I earn more money" the answer must be yes - it's more a question of commitment to it surely?
Absolutely. There's nothing stopping anyone learning a new skill in their spare time or doing a distance learning course to learn new skills and gain qualifications.