Why Do Spiritual Teachers Talk in Parables?

There is a reason!
Let's say that you wanted to teach someone to cook an omelet. One step might be to say, okay, cook me one. If it were me I'd open the fridge and get the eggs, go to the stove, and proceed. Whether you liked the way I cooked it is beside the point. Step one for me would be to get going on it.

What if you were working with an Indian who just stepped out of a lifetime in the Amazon Rain Forest? Different story, yes? At the very least a five minute operation would be a five hour operation and might be a five day operation. So much information about appliances, electricity, utensils, the house itself, on and on and on, would need to come first. If you were very thorough it might take months before that individual could cook an omelet with the full backdrop of understanding to really know what he/she was doing.

And this is with a solid demonstrable physical model.

Now imagine you were trying to tell such a person about making an omelet with no stove, no fridge, no house, no utensils. You are sitting in the rain forest with the Indian telling him about cooking an omelet to the point where he could walk into a house and do it. How likely is that to happen?

So it is with a spiritual teacher. The fact of spiritual growth and understanding is quite "real". It happens. But most people are working from a context of spirituality completely foreign to the actual model. It is well outside most people's reality. This means for all intents and purposes that it does not exist for most. 
A fact of understanding is one must be willing to understand. A second fact is one is actually moving in space for understanding to occur. One is moving into a "new" viewpoint. This takes some doing. No one can do it for you. And since you are moving into something that does not exist for you, you don't know that it's there. You have to have faith it will be there when you get there. Otherwise, where are you? In a sense you have to be an Indian sitting in a familiar rain forest with complete understanding of your environment listening to someone tell you something completely off the wall and unreal, and then being willing to walk into a kitchen and cook an omelet!
That takes courage. Or maybe necessity. But there is a space to move into. It is there, as a matter of fact, right in front of you. Growth is a process of spaces and each one appears as you are ready to move. No one of them will open all doors. But it gets easier.
So parables are used because of the unreality that we face. There seems no sense in telling an Indian to cook an omelet. But if you tell hungry people they can eat, they might begin to create a kitchen complete will all the tools they need. And everyone who wants to can do it. Parables are a means to get the wheels of understanding turning so people can make that kitchen.