but if everyone did that (probably half a million houses) it could save about 1/4 of a billion gallons of water over a year?
From your perspective the water is wasted, sure. But water is a nice stable molecule and whatever goes down the drain is promptly reused by someone downstream; as the city upstream of Dallas (Denton) likes to joke 'Dallas has water to drink because Denton flushes its toilets'.
It is cheap now, for us. For many, water is a precious commodity, not to be wasted.
Why waste it if all it takes is a few extra minutes to conserve.
If you want to do something like capture water that would go down the drain waiting for the shower to warm, have at. But like my tendency to promptly use ice in left over fountain drink cups to chill some water out of the tap the savings are extraordinarily small as opposed to more meaningful changes - like
not flood-irrigating thousands of square miles of lawns or not refrigerating one's dwelling down to 68F in the summer.