SharePoint Online - Retrieve an item - Set the "SharePoint site URL" by variable

Hi team,

I am using the "SharePoint Online - Retrieve an item" action in a workflow. Why can't we set the "SharePoint site URL" parameter by using a variable?
The "SharePoint Online - Update items" & "SharePoint Online - Create an item" allows for us to insert a variable.

This is extremely useful when switching between Dev and Prod sites. Is there an alternative option here? 

Thanks,
Bradley