Because SF is a town, not a city. And with the cost of doing business only on the rise there isn't room for places to be open during non-peak hours anymore. The number of coffee shops and restaurants that stay open later has noticeably decreased in my 19 years of living in SF.

Believe me I wish it were different but there are diners that are open late.

Dave Chappelle goes to Grubsteak when he's in town and they're open till 4AM. Or you can go hang out with the USF students at the 24hr Starbucks over in Laurel Hts.