The real question is why they don't cross paths more often.
Karen Starr runs a major corporation. Dick Grayson is the first son of billionaire businessman Bruce Wayne.
Nightwing is a respected and well known hero with ties to Gotham, Metropolis, the League and the Titans. Power Girl is a hero with ties to Metropolis, the JSA, the League, and Harley Quinn.
Dick and Karen already know each other and have worked briefly together in the League. They know a lot of the same people. There's tons of places where they could run into each other; business conventions, hero Crisis events, Haly's itself. They could run into each other while chasing down the same villain, at a memorial for fallen Terras, anywhere.
The real challenge is building their dynamic in a organic way and not instantly pissing off the hardcore Babs and Kori shippers.