You can invest in them, of course. I think the confusion is that fixed income may serve two different purposes.<p>According to Markowitz, you buy fixed income as "risk free" asset, to offset an optimal portfolio of risky assets. That way, once you have a target combination of risky assets, you just need to decide that % to put on that, and you put the rest to the risk-free asset.<p>Government bonds are usually very low risk. Of course, they are never risk free, plus you hold inflation risk. But Germany, US, etc. have negligible risk compared to corporate bonds or emerging / distressed government bonds. So they are good to plug into the "risk free" basket of that methodology.<p>Corporate bonds (as well as some government bonds) are much more risky, so you cannot use them like that. But they can be part of the risky portfolio, as they give much more interest and volatility, with which you can earn more (or lose it!).<p>So it's a matter of treating them as a different thing as government bonds, and understanding that when people talk about fixed income, they refer to the low risk government one, unless it is obviously otherwise.