If its not a bug (and they indeed do not like to leave their territory) then they need to have a spawn/aggro radius (small circle), roaming radius (medium circle) and a territorial radius (large circle), instead we have spawn radius (small circle) and roaming/territorial radius (medium circle).

The circles are from that of the monster's actual spawning position. That way you could have both types of parties (roaming and camping) because why would you limit the ways people like to level? We have solo and party why not roaming party and camping party to increase the options?