Bottled water is a multi- billion dollar industry world wide. It is simply a mind set of people (who can afford it) that bottled water is better than the water they can get out of their taps because of incredibly effective advertising campaigns.
People take that mind set with them on ships. The cruise lines aren't going to advertise the fact the water they produce onboard is at least as clean as bottled water, because they can find profit selling the water to those with that mindset.