You should invest in low cost index funds for the long term - meaning retirement. Investing for retirement is what the vast majority of people should be focused on for their investments. You can take advantage of that with such things as 401ks, Roth IRAs, and Traditional IRAs (tax advantage retirements vehicles)
These are the funds I Like
https://personal.vanguard.com/us/funds/snapshot?FundId=0085&FundIntExt=INT
https://personal.vanguard.com/us/funds/snapshot?FundId=0113&FundIntExt=INT
I am in favor of a 60/40 split between those two funds. Lot of people like to include bonds as well, but I don't think those are necessary until 10 years or so before you retire
Why do it this way? Because the data consistently shows that no one can consistently beat the market. Do they sometimes? Of course, but when you take into account the higher fees it simply makes no sense to invest in an actively managed fund. You will lose out. Data shows that you will be far better off simply following the market
Why not stocks? Well, I think investing in stocks is more risky, a lot more work, and requires a lot more of your time. The benefit of this is that you can just invest in those two funds (or three with bonds), invest in those ratios, and forget about it. No thought required. No stress too (which is a huge plus)
Why invest at all? Well, inflation. If you don't invest your money will be worth less than it is now. Basically if you don't invest you are losing money. Also, retirement. You need to invest, and have a pretty sizable retirement fund to actually have a comfortable retirement. If you don't invest and only rely on social security, you will be living in poverty.
As for why I don't invest in bonds and plan on investing in them when I am 50 or so. If you are in your 20s or 30s, you should be concerned with growth. Who gives a shit if 10 years from now the market crashes? That doesnt impact you. You only get loses if you sell, and since you have 20 more years to invest, the market will bounce back.
When youre 55 or so, start investing in bonds because if the market crashes when you retire and you need to sell your investments to survive, well, you definitely want to sell your bonds instead of your stocks. That reason being is that the bonds are far less variable than stocks, so if they go down, they wont go down much. Stock market crash though? Well, you would be forced to sell at a super low price, and that is not good for your retirement