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?
Because this whole thing is not stocks. When bitcoin have it own bull run more people tend to buy altcoins as they feel confidence in the market and because they are looking to get profit they are going into altcoins as with altcoins you can double your money faster than with bitcoin. When bitcoin is moving down there is less confidence in the market and everybody wants to preserve their value they have gained and thus they tend to switch to big cryptos such as Bitcoin, Ethereum or even fiat currencies or USDT - tether to try to save their value from going down.
So at the end as bitcoin is the biggest out there it provides the confidence in the market.
because every development that occurs in Bitcoin, Altcoin tries to follow it and strives to be a coin that is always able to compete with Bitcoin and fight the flooding currents to move forward in the future. besides with its increasing development makes Bitcoin so expensive. and Altcoin is inspired to continue to pursue it.