Because they cannot imagine anyone doing a potentially money-making activity for free. They can understand "I'll do it for free until I find a buyer," but not "I'm gonna do this because I love doing it. If other people get paid for it, well, that's their right; it's not what I want to get paid for doing."

A lot of people really just can't imagine loving a hobby in a way that doesn't connect to questions of income. Especially not a hobby that *can* be lucrative if done the right way. They think that the line between "amateur" and "professional" is one of skill, not one of avocation.