Why do people care more about bond ratings than stock ratings (if they even exist at all)? For example, there are many bond funds that say they will only hold investment-grade bonds (typically higher than grade C on most rating scales), while there are not many equity ETFs that use a similarly prevalent "stock rating" as part of their selection criteria. At most, individual stock analysts give "buy"/"hold"/"sell" opinions on individual stocks. Why are there no stock ratings that are widely taken as seriously as the bond ratings issued by the bond rating agencies?
The primary risk of a corporate bond (other then government interest rates) is the risk that the bond defaults, leaving you with at best a fraction of the original bond amount. The ratings are meant to be a rough probability of the bond defaulting.
Stocks have much different risks, since they are tied more directly to the performance (really the precieved future performance) of the company. So while bankruptcy is a concern for equity holders, they are much more concerned with the growth, efficiency, and future prospects of the company.
So analyst ratings (buy/hold/sell) are much more subjective and are less reliable (historically) than bond ratings.
Not to say that bond ratings are completely objective - they are based on projections and performance as well, but tend to change much more slowly than analyst ratings, which makes them more suitable as index criteria.