If only we had plants there ... but what those are nothing but "piles," at best. Nobody knows what's inside.
The difference is huge. In a normal reactor the fuel is carefully spaced, interleaved with moderating material, and has channels for the coolant. In a failed reactor we have none of that; the fuel may be wherever it wishes (at the bottom, likely) and the moderating material is somewhere on top, where it does nothing, and the coolant is nowhere in sight, and the heat-producing area is a small pool of white-hot lava. I have no idea where the Japanese reactors are on this scale, but clearly not at the "perfectly functioning" end of it.
Point is that the decay rate is exponential, and will decrease over time. That’s the only thing generating heat.