@Colin MacMillan When Blanket Orders were originally implemented, the focus was on delivering the core functionality first, so support for Related Items was not included at that stage.
If this would be useful for your business scenario, please feel free to submit a product idea or create a support case so the request can be reviewed and tracked.
In addition to the above, Blanket Orders are primarily designed for planning and release based ordering rather than interactive item selection, so features like Related Items are not included in that workflow. As a workaround, some customers use standard Sales Orders for item suggestions or handle related item logic outside of Blanket Orders depending on their process.