Correct.
Not everyone is doing content production. However, many independent webmasters you have your operating costs on top of your living expenses. Some of those could include...
1. Models. Photographers. Assistants. Labor.
2. Locations (for your photo shoots, or sessions).
3. Bandwidth/servers.
4. Development costs (coders, designers, etc).
5. Advertising, PPC, banners, etc..
6. Insurance.
7. Equipment.
Obviously some of those are monthly recurring, and others are going to be one time purchases you need to factor into your annual budgets. But you can see how there can be a lot of expenses, even if someone were making $10-20K a month, and depending on what they are shooting, a lot can be gobbled away before you get into the NET or PROFIT.
That said, even if you were making $10K a month (for simplicity sake), and $4k went to models and the rest, you still have (based on your example) $3K left over on top of living expenses for savings or reinvestment.
This does not get into other one time deals not included in monthly recurring, but that would fall under capital gains on your taxes. As the tax rate is much cheaper than counting it as income.
That said, there are many ways you can slice and dice this depending on the person's forte in this industry and what they are doing. Some are not going to have as many expenses, and no labor. Where others have that, and then some if they are growing or developing. Like coder, and designer fees additional.