It's a well known fact that very novice traders have a written trading plan. Every published study of day traders shows they make the same types of mistakes as the causal trader/investor does and the vast majority lacked having any sort of trading plan let alone understand risk mgmt, expectancy, risk mgmt, the road to ruin, etc. Most make a plan after having blown up at least once or more.
Of course you can make some big gains when just shooting from the hip, I sure have. I don't know why you find that hard to believe. I have known many non-traders that have made some big gains in a short time frame - the hard part is keeping it.