
Because it's efficiency is very, very, very small and people don't want to build them on their ships.
I have calculated when resources we have to put in a solar still yield to us. We have to wait more than 40 cantrian years to receive some income of our effort.
I see two ways of repairing this state:
1) make solar still easier to be build (for example 10 grams of window glass instead of 100 gram) or
2) make output bigger (for example 10 grams of water per day instead of 2 grams)
EDIT: If we make output bigger maybe this will help with an overall lack of water/expensive water and it will response a little to suggestion of MakeBeliever who want to encourage sailors to work with the towns. Sailors will have water to sell - not much, but always sth

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I think that people will be more eager to build solar stills, when we will "repair" them.
P.S. Sorry if that kind of post was somewere else. I just couldn't find it.