Why? Because otherwise I could not afford to work on it. Who would pay for my food for example?
Making money as an indie developer is really hard. Did you know that 5% of 2 million Apps are just profitable businesses? Just to make things clear: I can't pay my rent from Donut Dog so I need to work somewhere else.
It is sad that I need to write this in a FAQ section. And it is weird that people think that every digital product or service should be free.
So I ask you: Who should pay the people behind the screens programming stuff you use if you do not want to pay?