I live and work in the downtown part of San Jose, and on some days of the week work in downtown SF (SoMa near Caltrain).
I rarely spend $12-15 (more so in SJ but also in SF). Usually the significant cost is adding a soft drink of some sort (soda, juice, tea, etc.) because I'm finding restaurants are increasing the price of those for whatever reason ($2.50 at some).
Definitely wouldn't say $10 for a full meal is "unrealistically low". Perhaps it's really a matter of choice. Some people prefer to get lunch often from New American restaurants and similar where you need to tip, etc. Some don't mind going to simpler places like a taco truck around the corner, or Panera Bread/Chipotle/etc. (although I will easily admit Panera's cost has increasingly gotten more expensive, as you describe).