I'm sorry I can't give you a scientific explanation for your question, the most likely reason is that we are not comparing our dream to walking reality when we dream. I mean if our brain was to scrutinize every image that popped into your dream we would be lucid everyday.
I personally believe that our brain goes into this state of accepting reality because there is no point in questioning it. The brain is only processing the memories trying to see if they should be stored as memories however it is slightly more complex than that. It makes sense that our brain doesn't check if each thing is possible but it is also unusual how we rarely compare any of our waking expectations or memories to dreams as they happen.
Another reason is that we never question reality in walking life so we have no reason to do it in our dream.

You say that you feel that you cannot DEILD, I think you mean DILD. Dild is initiated in the dream while Deild is initiated by falling asleep consciously.
With practise and questioning your environment in real life that behaviour will pass on into your dreams, it reallly isn't much of a problem since youll get better at it.