This is normal in the MMORPG genre. The rationale is simple.

If the enemies never leave you, then you can't run away from them and are forced to fight them. This is a problem if you don't want to fight them and just want to run away. To prevent this, they go back to their origin if you get too far and stop attacking you.

The downside to this is, as you said, sometimes a mechanic will cause them to stray too far and return. The upside is it doesn't take forever to get away from them.

There is a content in this game where they take a very long distance to stop attacking you and that's Bozja. You have to run so very, very far to lose them and it's extremely annoying, because they aren't quick to kill. You can't mount while fighting either, so you're forced to run across half the map just to escape them.