We don't do it directly, but there are certainly municipal water supplies that are downstream of other town's waste water treatment plants.

I wonder if anyone has done an estimate of water losses in a home/day to evaporation (e.g. toilet bowl evaporation, humidifiers, drying laundry, drying dishes, etc.).

R/O is expensive. Tons of opportunities in the average home for re-use. Can use that shower/sink/laundry/dish water for toilets. A selective automatic bypass system could save the (mostly soapless) water to replace plant/garden watering (though that’s evaporation!).

A fairly large amount across a whole city, but compared to the amount of water run down the drain after use not a lot. Makeup water is needed for sure. However watering the lawn is one of the few that are significant enough to matter.

At a cost, cheaper to just get fresh water I would imagine

In the US at least if you are not treating it to drinkable the epa will stop you. Though where the epa tests is important, some times that means after it percolate through soil.

You do loose some of the bacteria, minerals, and trace elements though. But then again, those can be put back in with some ease. The problem comes when, like in Flint, the government decides to do really dumb stuff. I would not put it past some idiotic council to just pump raw sewage back into the water supply to save a dollar.