Because the main concept are all heroes are young and new, only 5 years into the game. They're written as mid twenties. Bruce is the outlier because they want to keep his kids, but Catwoman's following the others like Clark and Diana. Even Ollie, who should be a bit older since he's had Roy, look and sounds like a mid-twenties, and they kept Roy from Green Arrow book for most of the run to maintain that illusion.
She's 23, Dick's 21. It reads like Catwoman's new to the game, and really horny for Batman, while Dick and Bruce's relationship is like an older and younger brother. Bat and Cat didn't really have a relationship beyond sex buddies, and Catwoman didn't know who Dick Grayson is and his relation to Batman. She's seen and heard the Bat family but didn't know their specific relationships. Dick and Selina don't really interact, even when Dick rallies all Bat allies and she answered, they don't really have a face to face.