Because those folks do not understand how many things are set up before they jump into an "opportunity" or a "task." A LOT of people make the same mistakes, good for you for wanting to see the mistakes and avoid them. When looking at opportunities and tasks for example, you will see there are severe limits on what you can earn because of low pay or even an MLM setup in certain things.
There are a wide variety of ways to earn money online and many people do earn their incomes solely from doing so. Now let's get into some of the reasons that people don't make much cash. Many people do not understand the limits to many "opportunities/tasks/writing jobs" that can be found online. Many online opportunities are just that, they are opportunities that require a massive amount of effort to earn money. For those with fixed prices for certain tasks being done, the freelancer is simply being paid a low wage. I've looked at some of the rates people are paying others to write on certain sites and the amounts are pretty insulting.
For those who have an employer and may be working from home, sometimes this set up reveals the employee doesn't actually have the discipline required to do the work that needs to be done. Perhaps they have visitors, choose to do other things such as go out, lounge on the couch which will affect their output and sooner or later, their income.
There are companies such as Apple and many other highly reputable companies that hire people to work solely online and pay a fair wage. If one truly wants to work from home, they need to be as disciplined as they would be if they had to work outside the home and they need to look at the wages and think about the time required to meet their needs prior to diving in.
However, with awareness, patience and a great deal of effort one can work online and earn a lot of cash and one can most certainly CREATE their own product or service, create their own business, and earn a lot of cash online.