Why would it be a good idea to mix stocks and bonds in an investment portfolio?

Nearly Every investor will tell you to balance your portfolio. All in stocks in a recession will lose you a fortune, like All in bonds will let you lose, but not as much in a Bull market. 

In my opinion it is not a good idea. A stock has a much greater potential to give you a much larger return. You just pick the right ones and youre golden. Learn how to understand the fundamentals of a stock, choose what looks good and you arent likely to lose.

So that if either stocks or bonds go down (but not both), you don't lose as much.

There have been times when stock market performance wasn't that great. 

For me?   No.   I have no interest in bonds.

For someone else?   Sure, depending on their investment goals, risk tolerance, time  horizon, etc.   Many people feel they need bonds for income or diversification or risk abatement.

Because 100+ years of data on millions of investors shows that people who diversify their investments across different asset classes will make more money and lose less money in the long run than people who put all their investments in one type of investment.

If you don't want to make more money with less risk of loss then don't follow this advice.

Lots of people do it. Or you could own a fund that does both.

Like FBALX.