Are you asking why people invest in equities? It's because the market as a whole grows over time, sometimes as high as 15% but at present about 4% after taxes, fees and inflation. This means a buy & hold investment returns a reliable long-term averaged 4%, at a time when virtually no other investment can offer that kind of return (when expressed in the same way).
The above is true if one simply invests in an index fund that tracks (for example) the DJIA, and no trades are made -- no brokerage fees, no taxes until a redemption takes place, and lower taxes than for regular income.
Or are you asking why people have managed portfolios? It's because of widespread ignorance and a bit of the cowboy mentality -- a real cowboy would throw the dice instead of playing it safe. But "throwing the dice" in equities is easily shown to be a foolish act, historically, statistically, and reliably.
The average Joe does not know that.
I want to clarify that there's no real security in equities, my use of that term is only relative to other kinds of investments. Everyone should understand that you could lose everything, and there's no deposit insurance or anything like that.