There are many reasons, they really want to work on their project and also help people. It can also be for their own interest such as stealing investors' money. More so in our world and we would just be stupid to hope that their token increases the price and we can take back the investment of nearly x100.
Although I agree that a lot of people make altcoins because they want to help others and are generally interested in improving currencies and transactions as we know it, we should not also forget that some people are using it as a way and a means to make quick money off of unsuspecting investors who are just trying to make money in this market. This is not acceptable