Why is Communication So Hard?

Often when people call for counseling and I ask them what is the matter, most of the time I hear, “We just can’t communicate.”
This is a catch all phrase that means we don’t understand each other and it is hard on both of us. I get this, especially since I have been working with couples for 20 plus years.
I know that there is a person who feels very deeply about life and another person who thinks almost exclusively in their head. I have seen this in every couple I have ever counseled.