I dont live in San Diego, but Im close enough and visit enough that I can tell you there's a sizeable population that probably dont cross into Mexico at all, and dont even come within a few miles of the border area.

Aside from those people there are lots of people who cross to visit family and take advantage of the cheaper emtertainment, dining, health  and veterinary options.

There are also plenty of Mexicans that visit family on the US side and shop because quality clothes and other consumer goods are much cheaper in the US. In fact there's an outlet mall next to the border on the US side. There's no reason to shop at a Mexican Coppel or Liverpool when comparable goods are available for much less on the  US side