Perpetual motion does not equal infinite useful energy. To take the tied off gateway with a water source for instance. It would certainly be useful. But it wouldn't mean infinite energy. It wouldn't produce more energy than the Niagara Falls for instance.

Further, who in WoT would benefit from such technology. The AoL didn't need such devices since they had much better power sources. The current age doesn't have the electrical generators or scaled up mechanical devices to take advantage of it.

Finally, history is full of societies that using completely different technology than what we did and that did not use some of the most 'obvious' technologies from our point of view. The ancient Egyptians built the pyramids without the wheel or rolling of any kind. The wheel wasn't invented until centuries later. They simply never thought of using something like a wheel. It wasn't part of their technology. IIRC, as well the Chinese had gunpowder and firecrackers for a very long time before someone thought to use it as a weapon. It would be quite easy for us using our technology to think those people are idiots. But they are just as smart as we are, they just used completely different technology, much of which is lost. Some of that lost technology, too, allowed them to accomplish incredible feats such as moving and lifting massive stones that using today's technology would take millions (maybe billions) of dollars to do.

RJ is not without his flaws in realism, but I don't think that the lack of perpetual motion technology is one of them.