About 18 years ago, fresh out of Uni, I decided that being a commercial diver was going to make me rich. Accordingly I paid 4,000 pounds to do a three month air diving course at Fort William, Scotland to get the basic qualification to become a diver on an oil rig. After failing to get a job I eventually ended up in IT which paid off the debt, then eventually paid for the flying. I'm now flying a turboprop in New Zealand.
Diving has many similarities to flying. You paid for the course which gave you the equivalent of a CPL. If you knew people or were very lucky and networked very hard, you might get that elusive first job. It may have been overseas (eg the Gulf) or what was called civils (civil enginerering - long hours in shallow water.) Then, as your experience built and you made contacts, the jobs would come easier.
After a couple of years you could train to become a mixed gas (saturation diver). That was where the really big money was. In those days the peak in the North Sea was over, but even then for a month in saturation you would earn 10,000 pounds (limited to 3 months a year, although you could do air work in between on about 4,000 a month if I remember.
But, very similar to flying. You had to know people to get that first job. You could increase your prospects by buying more ratings (eg non-destructive testing) but it still boiled down to being in the right place at the right time - if you knew somebody they would ensure that you were in the right place at the right time, but there were lots like me that just couldn't get that first job.
So, flying isn't the only career where you pay for training, but it is far more expensive than diving, and much harder getting the qualifications. No wonder within a few years of getting your first airline job, most pilots become bolshie and militant. Having worked so hard and paid so much, we are now not goig to work for free and are going to use our muscle.