There's aesthetic reasons (I'm proud of my car, I like it to look nice), and practical reasons (Leaving mud, grit, salt etc, will wear and corrode my car faster).

Where I live, summer rain doesn't really clean the car (if that's what you care about), and in the winter you are shortening your cars life by years if you don't try and get the salt off regularly.

I rarely wash as well. But we're in a minority in US society.

Or a garden hose, bucket of soapy water, and a sponge. At least if you have a house with a garden hose. Many apartment complexes offer the amenity of an area for washing cars though.

Rain tends to just move the dirt around, at least where I am.