Because you need to sell something to be able to make profit.
Selling online is one of the ways people make money online.
There could be several means or ways a person can use to make money online.
Some were successful online marketers or sellers. While others are online teachers/tutors and making a lot of money from it.
Some bloggers and writers are also earning their living online through their sponsors, clients or ad commissions.
And sex cams. 🍌😈
I think content generation is the way people make money online. One way is to do YouTube videos, have a facebook or instagram page and slowly build a fanbase for your page or content. Then you can start monetising your content.
Other way is to be a blogger and insert ads that help you make money. You can try freelancing websites such as fiverr to find work that helps you make decent money for certain projects and not be tied to a desk job.
Then there are sites like musing and steemit where you can write and comment and hope to make 10 to 15 dollars a day. or you can invest some money in steem and power up and self vote your content to make money. the last one is not ideal as that will be considered abuse but if you are writing quality posts that others are upvoting, I don't think people will mind your self upvotes that much. share the love with others by upvoting useful comments as well.
Those are ways i can think of, i think content is the biggest money maker. If you have something different and useful, then you've got yourself a winner.