Retail investors are often served with standardized investment services because they:

Retail investors are typically served with standardized investment services largely because they outnumber institutional investors. The sheer volume of retail investors allows financial service providers to develop and offer products that are easier to market and manage on a large scale. Standardized services help in simplifying the investment process for individuals, who may not have the same level of expertise or resources as institutional counterparts.

By catering to a primarily larger customer base through standardized offerings, firms can achieve economies of scale, making the delivery of these services more efficient and cost-effective. This also helps ensure that the services provided are easily understandable and accessible, which is vital for those who may not have extensive financial knowledge or experience.

In contrast, the other options do not accurately reflect the primary reason for standardized services in the retail sector. The level of knowledge may vary among individuals, and while revenue generation is important, it's the larger population of retail investors that shapes the approach to service offerings. Additionally, preference for complexity does not align with the typical needs of retail investors, who generally seek simpler, more understandable investment options.