Both rifts and collision zones exist both on continents (thick crust) and out at sea (thin crust). Perhaps your point is that if left in operation long enough, collisions lift and thicken the crust and rifts thin out and lower it, so that, for example, the African rift valley would end up under water if it were to continue spreading long enough. But giving that as an answer to the question of how we ever got whole plates of such different thicknesses in the first place requires, on the flipside, that collision zones must have once created entire continental plates (or one supercontinental plate that later split up) instead of just pushing up little edge islands...