For one, thing companies and stocks are perpetual while bonds eventually fall due and can be renegotiated, and thus stockholders are kind of stuck with corporate returns. In that sense, stocks are considered to be riskier.
Well, let's think about it logically. If I remember what I learned accountancy college correctly (by no means a given) there are basically five ways in which companies can increase their returns, those being:
Over the decades the beancounters have used accounting trickery such as last in first out accounting (LIFO) to reduce corporate earnings and therefore tax payments. But over time, there is unlikely to be much of an improvement here either.
In any case, this is all working on the basic assumption that you are buying shares at the equivalent of book value, which in most markets is not possible, so this is a consideration for future shareholder returns too.
Simply put, you need to find a property investment which increases in value (or price) ahead of the inflation rate if you want to get ahead.