Well, I guess it's also because Marseilles, were Castille soap originally comes from, is on the seaside, so they could have water (even if sea water) for free.It is interesting that the original recipes called for using natural seawater. I would imagine that the natural presence of Ca and Mg, which make up the components of "hard water" and bind to soap molecules making soap less effective, are in effect probably working against the "slimey" properties of castile type soaps right in the bar.
Happy Bubbles,
Stéphanie