All currencies fluctuate against each other, but in most cases the currencies all fluctuate downwards in value within very close proximity to each other. You only see cracks start to appear when one currency becomes much weaker than the other ones. Look at the Japanese yen in the most recent days for an example. The big difference is that with the major cryptocurrencies the supply is either capped or very seriously constrained so it actually goes up in value while every other fiat currency in the world goes down in value over time.