There are plenty of plants that don't need constant watering, and will actually thrive in the hot Southern California weather.
Look at a Google Earth view of any of the wealthier SoCal suburbs. Almost every house has a lawn and many have pools (that are rarely covered to prevent evaporation). What an incredible waste of water in a state that doesn't have any to spare.
It's not a coincidence that LA County uses 80% more water per capita than San Francisco County.
That may be true, that there are plenty of plants.
But once you accept that aesthetics is a valid concern, your argument becomes one about the particular choice in plants, of which you conspicuously do not provide examples. Like cacti surrounded by bark or rocks (how friggin' ugly.)
Because when you actually start naming plants, you will quickly find that your solution is not quite as simple as you would like to think it is. Either the plant is not actually aesthetically better (not in your opinion, mind you, but the opinion of the homebuyer), or that the plant does not grow quick enough, or it cannot be supplied in sufficient quantities and sufficiently low cost, or it has high maintenance costs (think lawnmowers), or that it does not sufficiently prevent soil erosion like grass and needs additional landscaping (like bark or rocks), etc.
Identifying a problem is not so difficult. (Open up a math book.) Identifying a solution is often much more difficult.
Many humans are just not conditioned to live in an artificial, concrete jungle.