Why does light still come through my blackout curtains?

Almost always, the light is leaking around the edges not through the fabric. The fix is a wrap around curtain rod that brings the panels closer to the wall, or panels wider than the window so they can overlap. Top leakage can be fixed with a valance or a cornice box. If light is actually coming through the weave, you probably have a thin room darkening curtain rather than a true blackout with a liner.