Why we don't we save water by boiling the contaminated water to get the pure steam?
Boiling a water mixture doesn't always produce pure water.

Boiling a solution of salt and water (or evaporating the water) will separate the salt from the water, but those are compounds with very different properties.

In the making of distilled alcohols (Ethanol), the vapor phase has more alcohol than water.  Repeated distillations can increase the concentration of the alcohol.  But, the leftover undistilled solution is not pure water.  Ethanol and water are very "sticky" to each other.

Ethanol is just an example.  Household 'grey' water (waste water that doesn't include human waste) will have a whole mess of compounds.  Some you can isolate through boiling, some, like ethanol, will 'come along for the ride'.  Wastewater treatment is complicated.  Look at the system that had to be engineered for the International Space Station.  Actually, a version of this system has been tested for use in the types of communities you are describing.

Purifying grey water isn't a simple process.  There are, however, plenty of things that can be done with grey water.  Using grey water to flush toilets is perfectly fine.  With some simple amount of treatment, grey water can be perfectly suitable for watering crops.

So, since it's cheaper and easier to repurpose grey water, and harder to purify it back to drinking-water, reuse is more efficient than trying to completely treat it at home.