If it's not work and there is obviously high reward, why don't you go do it?
For the same reason I don't go into thieving as a profession (and I mean what's recognized as thieving by the mainstream, because moving bits of paper left and right and getting rich from it is currently an unrecognized form of thievery, just like being a company owner and taking for yourself 100% of the profits created by your employees' work while paying them only a fixed income that's considered a cost).
Certain systems allow privledged individuals to steal via markets and currency creation mechanisms.
This does not mean trading is inherently stealing. Without government sanctioned advantages (such as in the Bitcoin market), the only way to profit from trading is by moving assets from where they are available to where they are scarce. If that is stealing, so is the import/export business. And to accomplish that reliably requires a lot of work.