It would be very easy for flash to use QTKit to just play a video, in the HTML5 kind of way. But it would be quite a bit harder for them to do it in a way that remains compatible with all the different play/pause/volume/seek controls that different video sites have built in to their flash apps, not to mention the advertising schemes. If they weren't smart about how they handle all the compositing, they could probably end up really slowing things down when there are things like custom translucent controls overlayed above the video.