There are several reasons why people call or address bitcoin as a scam, it could be lack of much knowledge about bitcoin, or sometimes it maybe due to it high volatility of the market, like for example someone introduced them to bitcoin and the person was like invest now the price is low and that bitcoin will reach maybe $30k and so on, then fomo kicks in they invest and after investing the price of bitcoin starts going down, the value of their bitcoin in usd starts dropping they will definitely label bitcoin as a scam.