What does "fully explained" mean? By definition, an explanation is less accurate than the thing itself. An explanation that was without simplification or omission would simply be the thing being explained itself. Put another way, "all models are wrong, but some models are useful." An explanation is just a model.

Another problem with your formulation is that it takes a lack of knowledge, and just asserts that there must be something more than spacetime at play. But every phenomenon ever explained sufficiently has turned out to be "merely" spacetime. It would be foolish in the extreme to take an unexplained phenomenon and say "I know that everything else has turned out to be not magic, but this time, maybe it's magic!" Every single thing that keeps you alive and able to participate in our society - farming, medicine, the internet, cell phones, cars, airplanes, manufacturing, just to name a few - only work if the assumption is that the world works only via the cause and effect of physical processes. Every one of these fields requires millions of tests of that hypothesis a day. And every single one of those tests has come back as a success. Not once has anyone documented an instance where the cause and effect of the physical world didn't work. Out of, collectively billions of instances a day, not once.

So why would we, even for a second, take seriously the possibility that human cognition is the one exception to that rule? Especially when all of neuroscience increasingly can make accurate and dependable predictions that rely on the physical nature of cognition. To assume that we are special in that way, contrary to all evidence, would be the height of arrogance.