The theory behind lightning strikes is tremendously complex. You're asking "why people survive". It seems that you somehow think that lightning strike is by definition lethal. However, the question "why would someone die when struck by lightning" is even more interesting.
In a simplified view, you can think about lightning as of huge current source, and about our body as of resistor. In general, if the lightning strikes you directly and all the charge released during the discharge flows through your body, the damage will be severe. However, the damage may be localized to a particular area of the body - it depends how exactly the current flows inside the body.
The body may be modeled as a distributed resistors network:
The ultimate destination of lightning's charge is the ground, therefore you can estimate (very roughly) the path of current flow inside your body as a function of lightning's hit point. For example: if the lightning hits your leg, the current won't flow into your head, therefore the brain damage will be reduced (it is very simplified model! when someone is struck by a lightning, its whole body may become a temporary storage of the charge, in which case stochastic currents flow all over your body).
Until now we assumed that all the charge of the lightning flows through your body, but it is rarely the case. If you're wet, the resistance of the external layers of your skin is much less than the resistance of the body itself. This resistance is in parallel to body's resistance. It means that the majority of the current will flow "on the body" and not "inside the body". While this is still very dangerous (and may easily be lethal), the fact that the majority of the current do not penetrate into the body rises you chances to survive dramatically.
Any additional conducting path to the ground in man's vicinity when he is struck by the lightning also present additional parallel resistance for current flow and increases chances for survival.
Follow this link to find additional info on lightning injuries.