Because it is the easy solution.
It doesn't have to be that way and there are many parts of the world where two grades of water are piped to domestic properties.
You could do as we do, start collecting the shower water by putting the plug in the bath, and store it in a large bucket. This can be used to flush the toilet as necessary. We also have a fitment which goes into the outside drainage pipe, from the washing machine, and can collect the water as it is pumped from the machine. Into a bucket, and then straight onto the garden. Usually about 4 buckets full at least. Our washing up water goes on the garden as well.