Why is the email sys_glide_object not visible in the baseline instance?

I see instructions referenced in the community in numerous posts on how to modify the sys_glide_object write ACL to allow admin override, then update the email sys_glide_object record to mark it visible.  This so you can add a custom field of email type.  

My question is why is this disabled in the baseline instance?  Is there a reason ServiceNow has it set this way?

Thank you.