Because people don't really "use" water. When you flush the toilet or take a shower the water doesn't get destroyed. The vast majority of it gets treated and put back into the river.

The water used in agriculture will mostly evaporate or become part of the plant. Little of it will make it back to the source.

People absolutely use water by this metric, but most of it goes to their lawn.

Evaporated water eventually falls as rain, though probably somewhere else.