Why can't I search revenue line details from the Opportunity workspace area?

Opportunity revenue line information isn't available for Workspace search columns because an opportunity can have multiple revenue lines. However, if you've the Revenue Lines object enabled for Adaptive Search, then you can use the Revenue Lines workspace area to view the products and opportunities for each revenue line.

To view a list of opportunities having one specific product, you can add a filter on the Revenues field to search for corresponding records. When you create an opportunity, a primary revenue record is automatically created. The primary revenue record is independent of products added to an opportunity and is always associated with the opportunity even if no products are added to the opportunity. The primary revenue record displays as a summary line in the UI but no product related information such as item name or description values are displayed.