Welcome, Harold. From So cal semi-desert. I feel for you. And I gave up on lawns and went with a cottage garden.
I think there are several different types of grasses we call crab. Most of them are creeping and invasive types. Take a look at grass seed
and how light it actually is. It will blow in the slightest breeze and travel with pets and pests in their fur, etc. Now look around your neighborhood for types of grasses that are around. You can begin with a lovely sod but you will get contributions from all over.
So a lawn will eventually revert to what mixture is nearby.
The healthiest, thickest lawns can help reject unwanted seed
. You can stave off the invasions with diligent work and then reseeding patches often. One of the best types in SoCal for warding off volunteers is a healthy St. Augustine. But this is a grass that needs lots of work on its own. To keep it in bounds and green. And you need a heavy-duty lawnmower to cut it.