I found many sources that go against buying corporate bonds, because it's too risky/expensive. They prefer government bonds and shares. But investment advisers keep using them.
As for corporate bond being more risky/expensive than government bond, ask investors who invested in Greece Debt vs those who invested in Apple debt.
You may want to consider reading up a few books in investing and finance. /r/personalfinance, /r/investing, /r/financialindependence, and /r/finance are a few good subreddits with some smart regulars. Also these subreddits have good list of recommended books.
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.
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.
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!).
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.
I am sorry to say this is incorrect. No knowledgeable person will refer fixed income as "low risk government one". Fixed Income covers a broad range of assets that are typically debt-based, non-equity based that generate periodic payments/interest. This includes Government Bonds, Corporate Bonds, and other debt securities (Real Estate, Consumer Loans, Invoice Financing, come to my mind). Fixed income is a very large and complex (much more than equities) field and calling it as just "low risk government one" is a disservice to the field.
> According to Markowitz, you buy fixed income as "risk free" asset, to offset an optimal portfolio of risky assets.
I would like to see reference that attributes this to Markowitz. Markowitz's Capital Asset Pricing Model (CAPM) refers to "risk-free" rate but never specifies if fixed income equates to risk free. There is no risk-free assets. Anything close to being considered "risk free" is very short-term (not long term) government paper issued by US, UK, Germany, Switzerland and Japan.
Also:
> No knowledgeable person will refer fixed income as "low risk government one".
Being knowledgeable is not something binary, and you have lots of people in the internet talking about holding stocks and bonds. Whenever you see investment advice stating "hold your age as percentage of FI", they are not referring to junk bonds, nor securitizations or whatnot. They are talking about low risk, low yield, typically government bonds.
EDIT: Context is also to consider. In some contexts, people equate both things.
In any case, rereading my post, it now sounds a little bit defensive, so sorry for that.