Because the Categorize Layers command also has the ability to a) associate a Layer with a Site Improvement and b) flag that a layer "contains potential boundary lines as well as the ability to Categorize / recategorize layers to the Takeoff Categories there is more functionality than we would really want to burden the Layer manager with at this time.
I recommend creating all of your layers and layer groups in a prototype / project template and also categorize them in the project template so that you are only creating the odd new layer as needed to supplement your standard layers - this will make you faster and managing your project template over time so that "all the layers you ever need" already exist in the template as categorized layers with all appropriate settings.
While I do see that Layer Groups and Layer Categories would be used in a very similar way, they do have differences.
When we come round to a point where we rework this part of the software I am sure we will consider some consolidation here also
Alan
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