Because you are not picking an enemy instance. If you have multiple enemies, without picking, the distance() expression will only check one of them.
I suggest using Line Of Sight behavior instead - add it to Enemy sprite, set distance to 150px.
Enemy has LineOfSight to Player : Player subtract 1 HP
Also, if you have multiple enemies, disabling their event groups is not a good idea. It will obviously disable all enemies, not just one.