I’ll nitpick that said definition is also arbitrary. Why is it 1l of water at sea level, and not molecular weight of the water? And why a Liter anyway.
Even metric units like time are somewhat arbitrary. Why is a second based on caesium frequency, and not some other element?
The short hand answer I’d try to give people is ‘it’s statistics’. Based on training data, there’s a certain chance of certain words being in proximity of each other. There’s no reasoning behind placement, other than whatever pattern is discernible from known situation.