It's the way AI works, i wouldnt go change it because the consequences could be dire (like nobody attacking you at all hehe).
I think it's made to conserve CPU time. If all monsters in the world would be thinking non stop about what they can kill, the server would lag a lot. That's why they only look around and think about what they see every 5 seconds. Or, when attacked.
About war mode - if you attack a monster, you can't be in peace mode i suppose

These are some standard UO things - it might be different on osi, but all shards that play RunUO have the same system.
+Colibri, Administrator of UO Excelsior Shard
Don't know what the purpose of your life is? Well then make something up!

(Old Colibrian proverb)