Explain why on average the profit levels for invention and entrepreneurship are generally so low

Although a new invention initially implies a higher rate of return at or near the level of monopoly profits, eventually additional competitors will enter the market producing substitutes. This competition implies that economic profits will move toward zero. In addition, there are a number of inventions that ultimately are not successful. These facts coupled with the excessive optimism of entrepreneurs and non-financial, psychological rewards for invention lead us to expect a low or even negative overall rate of return on invention.