I'd also prefer a natural explanation for respawns, even if it was just something like portals from other locations, or migration from enemy strongholds, etc.
The problem with respawns outside is they just get annoying if you need to run back and forth somewhere, stopping continuously to kill the same creatures over and over. Generally for caves, you don't need to keep going back, so respawning isn't as much of an issue.
Welcome to the forum.
