Catwoman is not a "hobby" any more than Batman is. To be what he is, Bruce had to spend his entire lifetime training. Because top-twenty martial artist, world's greatest detective, chemist, inventor, knower of insane and obscure trivia is not something you can do "on the side." Selina, likewise. Her talent as a thief, the depth and breadth of her safecracking skillset, is not something anyone could ever achieve incidentally, while focusing on something else.