Why do some people think bitcoin is a scam? What do they mean by this

I get other alt coins being a “scam” but I don’t understand what they mean when they say it about bitcoin. I mean its shown to have real use, and its gotten pretty big especially with the etfs

I don’t know a ton about bitcoin but I’d like to learn why some people think it’s amazing while others think it’s a complete scam. What exactly do they mean by “scam” what do they think will happen to it? Do they think it’ll just crash and never recover?