The reality is that I just don't have the mental (and sometimes, physical) energy.
I have found that mental energy is just as important as physical energy when having a dialogue. Phone conversations force me to listen to, process, and respond to what the other person is saying. Email correspondence requires me to read, process, and respond to what the other person is saying. I do so much of these mental aerobics at work that by the time I get home I just don't have anything left to give. Unfortunately, I'm not much better communicating with people on the weekends because I usually spend most of that time trying to recover from the week (i.e., sleeping).
It's not personal. It's CFIDS.