There are lots of ways OpenCV complements Tensorflow, pytorch, and other high level frameworks. They don't need do all those things themselves, because they assume you have OpenCV around for stuff like image io, preprocessing, manipulation, transformation, color space conversions, edge detection, filtering, segmentation, morphological transformations, feature detection, video io, real time video capture, camera calibration, 3d reconstruction, optical flow, object tracking, qr code detection, gui creation, etc.