I don't know why people say bitcoin is gambling. Its not gambling. You are investing your money and time. You use your internet. So its not gambling dealing with it.
Bitcoin is a currency. You cant say money is gambling itself. But if you invest your money and convert it into bitcoin, now it is gambling. Because you take a risk investing your money to a volatile currency hoping it would rise up in the coming days, but you know it might also drop. There is still win or lose in it, so basically its still gambling. The process is gambling, not bitcoin itself.
I think their where the idea that bitcoin is comparable to gambling comes from the idea that its value is volatile, and that you are gambling in the sense that it can be higher or lower, therefore, there is some sort of an "unknown" aspect about it like how gambling goes relying on the "chances." However, I agree that it's not the same because although its value can change in a split second, it's not because of change. There are market trends and happenings that may effect such change.