Because they're not really meant to be triggered. It's like a turret map or whatever, you would need to trigger them all for them to spawn. Which is stupid.
Hey creeder Do this,make a start trigger,then make a name for the building you want,then put the name of the building into the trigger box for the start trigger.Once You've done that you can use any kind of trigger entity for the building!
When you press button which will create Turret(for example), you will see it.
But others won't see it and when turret shoots them they'll just lose HP and won't hear any sound of turret.