Yes, adding new mechanics for players to tackle is what most of us would like. However:

1) It is a lot more costly to think of, implement and then test new mechanics than to simply change the value of a variable.
2) Let's assume they implement one super amazingly fun mechanic that feels extra rewarding to play correctly. If the enemy that uses it dies in about 0.15sec from the moment he spawns in the map, is this new mechanic worth it?