The game is not zero sum. When a friend and I chop down a tree, and build a house from it, the house has value, far greater than the value of a standing tree. Our labor has turned into something of value.
In theory, a company starts from an idea, and offers either a good or service to create value. There are scams that make it seem like a Vegas casino. There are times a stock will trade for well above what it should. When I buy the S&P index at a fair price for 1000 (through an etf or fund) and years later it's 1400, the gain isn't out of someone else's pocket, else the amount of wealth in the world would be fixed and that's not the case.
Over time, investors lag the market return for multiple reasons, trading costs, bad timing, etc. Statements such as "90% lose money" are hyperbole meant to separate you from your money. A self fulfilling prophesy.
The question of lagging the market is another story - I have no data to support my observation, but I'd imagine that well over 90% lag the broad market. A detailed explanation is too long for this forum, but simply put, there are trading costs. If I invest in an S&P ETF that costs .1% per year, I'll see a return of say 9.9% over decades if the market return is 10%. Over 40 years, this is 4364% compounded, vs the index 4526% compounded, a difference of less than 4% in final wealth. There are load funds that charge more than this just to buy in (5% anyone?).
Lagging by a small fraction is a far cry from 'losing money.'
There is an annual report by a company named Dalbar that tracks investor performance. For the 20 year period ending 12/31/10 the S&P returned 9.14% and Dalbar calculates the average investor had an average return of 3.83%. Pretty bad, but not zero. Since you don't cite a particular article or source, there may be more to the story. Day traders are likely to lose. As are a series of other types of traders in other markets, Forex for one.
While your question may be interesting, its premise of "many experts say...." without naming even one leaves room for doubt.
Note - I've updated the link for the 2015 report. And 4 years later, I see that when searching on that 90% statistic, the articles are about day traders. That actually makes sense to me.