Because that’s just not how RADIUS Accounting works.
By the way, our integration with Active Directory (either AD Query or Identity Collector) fundamentally the same way: we “subscribe” to an identity source to find out about what users are associated with what IP addresses.
The gateway will perform an LDAP query to determine what groups the user is a member of to calculate the appropriate Access Roles for that user.
This way, at the time the user tries to do something through the gateway, we’ll know precisely what policy applies.
There isn’t a standards-based mechanism for either RADIUS or Active Directory that I’m aware of that allows anyone to query “what user is associated with this IP.”
Not to mention; you’d have to hold the connection while the lookup is performed, creating a performance issue for end users.