Why aren't we recycling grey water to flush our toilets?

This may be the wrong place to post this, I apologize if it is.

Why aren't we using bathroom tap water (grey water) to flush our toilets? Seems like a fairly straight forward adaptation that would have an incredible impact on water scarcity.

Take the grey water from the bottom of the bathroom tap, add a large particle filter, and pipe it into the top of the toilet tank.

What do y'all think?