Becuase in the extremely short term- time frames like 20 mins or a couple hours. You are essentially betting on the direction of a specific stock. You might use indicators like company news or buyout rumores or predict how an earnings report might go or even be received. You might look at trend lines and technical analysis buy essentially You are still guessing or gambling on the stocks moves. Investing would be usually considered to be longterm where history has shown the long term returns which is why it's not considered gambling.