The short answer is, it shouldn’t be. The longer answer is more nuanced, although the simple truth is that for some -- perhaps most -- people, investing is almost indistinguishable from gambling. (Spoiler alert: It depends upon how you do it.)
When a person is gambling, they are relying on an event’s random outcome over which they have little or no control. Whether it's slots or craps or roulette or Powerball, games of chance are just that. Other games such poker or black jack require some skill, although luck remains a key aspect of the outcome. The unifying issue across all of these is that the odds are stacked against the gambler, with the house invariably the winner.
If that description reminds you of your own portfolio, than you have an investment-process problem.
Your goal as an investor should be to eliminate as much of the element of chance from your process and, like the house, stack the odds in your favor. This is true whether you are actively pursuing alpha (market-beating returns) or passively accepting beta that matches the market.
How do you become the house? You:
* understand the nature of risk
* are comfortable with the idea of uncertainty
* rely on long-term measures of valuation
* use mean reversion as a guideline to unknown future outcomes