There's another reason.
When you start trading, billions, millions, etc. are just numbers.
As you start to actually make some serious money, your motivation to make much more decreases exponentially, whereas your fear of losing what you already have increases.
While new traders may become more aggressive after a lucky run, most experienced (and burn/bitten) traders become more and more conservative as they net more money. Thus, the trader that was not so happy with making 40% (or more) a year ends up being really happy making 15% with lower (perceived) risk.
And then compounding goes through the window.
Or as someone put it: it takes courage to be a pig. :eek: