1. Maybe because it's not visible, some people believe in what they can see ,feel and touch ,
This is possible. But one thing for sure why they call it a scam, they don't have enough knowledge about bitcoin. This is not about seeing it physically but more on how they see it. And since people prefer to judge something even without knowing anything about it, they can easily judge. That is the same in bitcoin, people who don't know about bitcoin is afraid of it and tend to call it a scam because they just wanted to avoid it for not knowing anything.