Hey Thuận, thanks for your comments.
You are correct, investing in stocks has no guarantees of generating profits and there is a likelihood that they will underperform, and even in some cases you could lose more money than you gain.
However, the point here is to mitigate against most of this risk.
How? By not picking individual stocks and putting large amounts of money into individual stocks, and hope for the best.
Instead, to invest in well diversified funds with 50+ holdings in them, made of companies operating across sector verticals, geographies and cutting across asset classes too.
Such funds, which can be very attractive with their costs for the more passive ones, can provide a great deal of diversification. Thus, allowing you to be in the market without losing sleep over it!
I hope this helps.