People get into this for many reasons, but I have to say that the biggest reason is to make money. Whether to try to make this a full time gig, or make just enough to make car payments, money is a big motivating factor.
Unfortunately, people get the wrong idea about how "easy" it is to actually make money doing this. In fact, some 93% of the people who actually consider themselves internet marketers do not make one red cent.
Part of the problem they were sold a bill of goods. They were told repeatedly how easy it is to do, that they can make money in their sleep, etc. What they do not say is how much work is actually involved to GET to that point.
Then there are those who simply do not want to make the effort. They buy product after product, open it and look at the work involved, and then stops on that day. Then they wonder why they do not have the success they feel they deserve.
At the end of the day, it comes down to focus and desire. True desire to succeed, then the power of focus to learn and put into action what they have learned.
It also helps a lot to laser focus on one or two specific things at a time. For example, focus on being a killer copy writer, or perhaps article marketing, or creating and selling products. In the world of internet marketing, there are so many sub-niches that would be difficult to focus on just one or two.