Well if you ask anyone who says bitcoin is a scam, how bitcoin works, they have absolutely no response because they have absolutely no clue how it actually works. I remember when people use to say this to me all the time back in the 2015/16/17 days. My first question to them would always be "Okay, do you know what the blockchain is, or can you explain how it works"..never once was any single one of these persons able to do so. So, there's really just a lot of noise out there.
I talk someone and asked him if he knew about bitcoin but surprisingly, he boldly said it was a scam thing. He told me that one of his friends got scammed from investing in bitcoin online by a stranger from social media offering huge ROI.
I understand his stance because of his friend's experience and some people are saying the same. Some people who said that bitcoin is a scam are those people who literally don't have an idea how it works, those people/investors who slip into trusting strangers easily without validating their offer is true or a scam. A scam never happens if we do our part which is to research before investing something. Unfortunately, most of us did this after becoming a victim.