The problem with stock market trading is that the need for liquidity grows with the size of the market, not with the number of traders. So if traders keep joining the market over and over again at some point there will be an excess of liquidity and there will be no positive sum money left to earn. It's like a real business. Once everyone has a car the only way to grow market share is by displacing other manufacturers.

If you want to make a living as a trader you are supposed to look for very volatile and iliquid stocks. Traders make money off the difference of the current price and the actual value of the underlying company and in volatile markets that difference is very high.