Trading the ES day in and day out is the reason why its so hard. In other words, trading 1 securrity (whatever it may be) is really what makes it hard. If you were trading IBM everyday, thats still just as hard.
IMO, most stock traders say they cant trade futures because they have a strategy that can only be executed by finding a particular setup or 2 or they need a certain volatility condition or whatever the case may be. And if they want to trade everyday they have to look to the thousands of stocks to find what they need to trade.
If you have a particular setup that you are confident in and are only trading ES, that may come around maybe once every couple of weeks or whatever, that makes it tough to trade.
So you either need to become really really patient and trade big size or a different kind of trader, a really good smart trader OR just trade stocks and find what you're looking for everyday.