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.
There is investing and then there is speculating. Bitcoin is a highly volatile and high risk investment.
A Bitcoin investment is when an investor understands the fundamentals, technology, and long-term potential of Bitcoin and invests into it with the intention of holding it over a long period of time.
Speculation is when someone buys Bitcoin and plans to hold it for a very short period of time with the intention of timing the market. It is a bet, might go in favour of the speculator, might not. So speculating is similar to gambling.
So people who say Bitcoin is gambling consider it as a speculative asset rather than a long-term investment.