I don't know those people.
The ones I know who do very well have either read plenty of textbooks or at least been taught in a formal setting by others who have.
I think it's true, though, that some people have an almost instinctual grasp of statistics and probabilistic concepts which eludes most others (I suspect it's closely related to how they were taught maths at a very early age), and that almost nobody ever makes a living trading forex without having, acquiring or developing that.
I'm never quite sure what people mean (especially in this kind of context) by "common sense": I sometimes think that it's far from "common", anyway.
It tells me something different ...
It tells me that (a) forex-trading attracts gamblers, (b) that the trading world is packed with businesses whose principle modus operandi is to take advantage of them, (c) that some people try to run before they can walk or understand money-management at all, and (d) that it's an illustration of the principle that the overwhelming majority of attempts at self-employment/self-employed money-making are destined to fail.
It's a curious thread: the OP apparently made a total of one post in this forum, and hasn't logged in here for 2 years; but some of the replies are interesting.