Actually the reason for this is, since Bitcoin of he first crypto coin, not all altcoins can be bought directly. For you to buy them, you first need to buy bitcoin and then swap to those altcoins. And before you can sell them, you still need to swap to bitcoin before you can sell. So its more like Bitcoin is the intermediary of those coin. So when the price of Bitcoin increase they also increase and when Bitcoin decrease they decrease, any trend Bitcoin takes they follow. However from my point of view of things, it seems they will experience the bull run with Bitcoin. Change is constant, anything can happen anytime so just be prepared for anything that might come.
this used to be the case back in 2016, 2017 until 2019. but now Stablecoin has taken over that spot from bitcoin. you don't need to buy bitcoin to buy altcoins anymore. u need to buy stablecoins first to buy any altcoin. you can still buy Bitcoin directly without buying any stablecoins. so Bitcoin is still strong in that manner.
Nowadays thanks to large p2p exchanges such as Binance, you can even buy a lot of different altcoins directly from people all over the world.
thats right, stablecoin replaced that intermediary, basically the reason why stablecoin has grown so much that china willing to ban it.
because people all over the world are using it because its just that convenient also no need to worry about volatility if you try to hold off from investing its really convenient these days.
even back then though i think people are buying altcoin directly using fiat, but represented as digital fiat in many exchanges right now with the presence of stablecoins that are integrated directly in the blockchain
things just getting more and more convenient, no wonder tether market capitalization increasing like crazy.