Well, according to the Star tabloid there's a very good reason for that: her psychic told her not to date. And Sandra supposedly takes her psychic very seriously.
The not-always-reliable tabloid claims the psychic told Bullock to focus on raising her son Louis, and not let a man come between them. A source is quoted as saying that in 2002 when she first started dating Jesse, "she [told Sandra] Jesse was bad news, but Sandy chose to ignore her warning. She'll never make that mistake again."