And you'd be right. All the compasses on Earth have never all spontaneously pointed in the other direction. Ever since humanity developed them, they've always pointed the same way.
This poses no problem for a several billion-year-old Earth: by accepted dating methods, the last
geomagnetic reversal happened almost a million years ago. Nobody was wandering around with compasses then to notice the change.
However, with a young Earth (and I'm classifying anything up to a hundred million years old as "young", here), we run into a problem. When you look at all the sea floor from the oldest parts to the newest parts, you can count about 200 reversals. One of the side effects of claiming that the Earth is anything less than 230 million years old (the age of the oldest parts of the sea floor, thanks to
subduction and
sea floor spreading), then you start having to compress all those geomagnetic reversals into a smaller window of time.
If you go by the YEC claim of 6,000 to 8,000 years for the age of the Earth, then you would have a reversal about every 20 years on average
as a bare minimum.
We know the compass was invented in China around 200 BC. Regardless of any other lines of argument about how rocks are dated and the like, we can be sure of one thing: there have been no geomagnetic reversals in the last 2,200 years, because people have been directly measuring the direction of the Earth's magnetic field that whole time.
Now... how young can the Earth get so that within the last 2,200 years, there were no geomagnetic reversals?
Hmm. It seems bizarre for me that if someone would throw out evidence, science, logical consistency and common sense to the point that they accepted young Earth creationism to begin with, they'd let little details like that interfere with their beliefs:
"people living to the age of 900, a 6,000-year-old Earth, talking snakes, humans being made from dust and/or the ribs of other humans, and a global flood is all reasonable... but a man living under the protection of an omnipotent God without a fully-formed ecosystem? That's just nutty." This mindset makes no sense to me.