Well, it is a very very simple reason why people do this ; money. Think about it, the costs of starting a new ICO or even creating something with just airdrops and not even get funding, means that you will be in control of a coin that is used and exchanged in many platforms. No matter how little you can spend, in the end you will just sell bunch of coins and you will make that money back and profit from it.
So, there is no reason NOT TO create a new coin, if you have the capability of creating a new coin, you probably should because there are hundreds of people out there who will most likely want your money as well. I personally won't because I know it is shady and immoral to do so but I do understand the shady and immoral people and why they do it, in a world where money is very difficult to come by, potential of making thousands of dollars is a good reason to do so.
That is the fact, moreover making tokens is not difficult. That is why many fraudsters can do this easily, making teams that appear to be authentic or deliberately making them anonymous, making various scenarios of fraud with projects that seem real. However, the fact that it is only a fraud scenario, some even really just want to raise funds without any real development. That is why we must be truly selective.