why did 1Stream design their workflow to start from / associated with a cube?

In order to sell this enhancement request to the Product team, I feel there needs to be more clarification on what you are looking for.  If I understand you correctly, you could currently do what I think you want to do.  Create Workflows with no regard to Scenarios.  There will always be an association with Workflow to a Cube.  That's because the Data Sources and Transformation Rules need to know what Cube Dimension are being used and identify the Dimension Members to validate against from 1 specific Cube.  Of course, this is assuming data records are being loaded to a Cube through Import, Validate, Load via Stage.  If you had multiple Cubes and are only using 1 Cube Root Workflow Profile, you could set up Workflow where it isn't by Scenario Type.  If I'm understanding you correctly, maybe you are referring to something like this?

Finance and Accounting Workflows would have separate Workflows that can load to any Scenario and Scenario Type ( assuming they have access to them ).  Only 1 Cube would be setup as Top Level Cube for Workflow and NO suffixes defined by Scenario Type.  Data Source and Transformation Rules would be assigned to the Top Level Cube for data loading.  Data Source and Transformation Rules can be unique by Scenario Type.  This could work for a single Cube design, which should only be used in rare occasions or Linked Cube design.  In Exclusive Cube designs, each Cube would require its own Workflow hierarchy so each Cube would be set up as Top Level Cube for data loading.  That means Data Sources and Transformation Rules for Imports that are specific to its own Cube.  In addition, adding another level of security to secure who can see/access which Cube Root Workflow Profile structure.  However, it could be set up the same way as in this screenshot.  If this screenshot is accurately depicting what you are referring to, there are pros and cons to this type of implementation.  Also, the Workflow controls all the data loading, clearing, and locking by Workflow Data Unit which includes Cube.  So Workflow and Cube are closely aligned.  This really goes back to LeeBown question on what is your process and what tasks do you want your Workflows to manage.