Do they really? I often encounter this statement. And i have no idea as to why. This past few days, bitcoin is on the rise and most certainly than not altcoin's price surely follows. I am not saying this is bad but altcoins have very little in common with bitcoin. And they are certainly not carbon copies of bitcoin either. Why the heck is their price so closely connected despite being so fundamentally different?
I think it's not always that way, because recently Bitcoin went up quite big but altcoin did not keep up with a comparable ratio. But the price will always have a correlation, because many investors who buy only wait when global market capitalization increases. And with the magnitude of Bitcoin dominance, I think the global market will be determined by the performance of Bitcoin.