Why? Because the volume of data available, and the variety of forms that it takes, make collation and analysis by competitive intelligence professionals challenging. Because these employees are already encountering this information in their jobs, in their personal lifes (e.g. as customers) and via contacts etc.
How? By making it very easy for employees to log that information on the intranet, via email, via app, etc. For example a wiki intranet site of competitor information that an employee can add to. The sales team might contribute the data on competitors' pricing, the research team would contribute data on competitors' technologies, the finance team would contribute data on competitors' financials, etc.