Stupid Question: Why do some cryptos move together?

I remember back in 2017 & 2018, there was a saying that when BTC pumped, the rest of the alts bled out and vise versa, but now it seems like some of the charts looks exactly the same, especially ETH & BTC. Why is that? Whales controlling the market? ETFs that have BTC & ETH linked? I'm trading futures on BTC right now and I'm just wondering what caused the change, and when did it happen? Probably a dumb question but I'm curious. I used to exchange between a few alts and BTC when the market moved to gain more (didn't always work out) but now it seem impossible, at least with ETH & BTC.