In his article about why it's hard to focus, David Rock explains that attention uses up brain resources, so it's limited (although more coffee sometimes helps). Also, there is always some kind of neural activity going on in your brain and that contributes to its restlessness and distractibility.
To compound these problems, the effect of distraction tends to accelerate. That means once you start giving in to an urge to, say, check email by opening your email program, it will be much harder to keep yourself from doing it than if you nipped that pesky desire in the bud.
What's the answer? You already know. You have to turn off the distractions. Stop fighting your brain's natural tendencies and work around them instead.
The other, more subtle, answer is to develop mindfulness. Rock has another fascinating article about that topic, where he shows that the brain has two different networks to experience the world. One filters through the self (attributing meaning to events, for example) and the other is direct sensory experience (being present in the moment, for example). It's not surprising to find that people who are good at being mindful have more cognitive control and thus can manage distractions better.