[digibri]

That's a good question. I'll take a stab at answering it. (This is supposition on my part, it is not backed by links to wikipedia or anything.)

I believe the word "arbitrary" applies because Magnetic North pulls a compass needle in a specific way, but that technically, the needle points both north and south.

It is merely due to convention that we navigate by first finding magnetic north. It would be just as easy to navigate by finding magnetic south...mathematically speaking.

[VLM]

Navigators had stellar nav a long time before compasses.

If you live in the N hemisphere theres a really convenient star nearly perfectly (by olden standards) north. Polaris.

If you live in the N hemisphere you could never see a "south star" if one existed, which it doesn't (although there are some close ones).

If you live on a rotating planet, there's no such thing as a permanently "East" star or permanently "West" star.

So you inherently end up navigating off the north star. And in another post I tried to explain there's some built in human-user-interface issues that you tend to hold the map upward pointing to the one feature you're sure of, and with ancient-style celestial nav thats always going to be north because of the north star aka Polaris... so once you have literate map users, map makers are inevitably going to write place names right side up with top pointing to Polaris aka the north star aka "North"