Why is a default rate not a good sole indicator of the potential performance of a portfolio

Question:

Why is a default rate not a good sole indicator of the potential performance of a of high-yield corporate bond?
Portfolio
A portfolio is a grouping of financial assets such as stocks, bonds, commodities, currencies and cash equivalents, as well as their fund counterparts, including mutual, exchange-traded and closed funds. A portfolio can also consist of non-publicly...