Because it's unpredictable you need to be as good as you can to reduce the chance of failing that's why it's such a difficult activity.
The trading is not profitable for most people. It is only for the professionals to take money from the unprofessionals.
I would like to disagree on some point, professionals and unprofessionals seem dont have any connection regarding on trading and you wont need to be professional to take advantage on trading as you could possibly engage to it as long you are knowledgeable to it and know analytical skills.
I think he meant professional traders vs part time or untrained traders. Those who practice trading for a living vs those who just started trading. Those professional traders have a better perspective and have more resources on trading so they can eat you alive on trading.
Yes there are some people who benefit most from the trade so that they can eat and drink from the trade. It does happen if someone already understand and experts from the trade, it also requires a long process. If I think that trade is not unexpected that my job is not dependent on trade. I also have a daily job and I also use trade as well to supplement my income.