There was a discussion on NPR not to long ago about this very topic.
In many countries, there is no running water or even a convenient
pond/well/whatever of potable water and here we have perfectly safe
water coming out of the faucet in the house and we choose to instead go
buy water(which in many cases is no better/different than what comes
out of the tap). Granted there will be exceptions to this(local
flooding, hurricances, etc) but they are typically temporary
interruptions in good tap water.

The commentator had a particularly clever summary statement about the
issue, but I can't remember it well enough to do it justice.

JW