Because economically that’s true. There is no crypto (even ETH) which could stay at its price when BTC goes up and down by a magnitude. Of course they are correlated, like INTC may correlate with APPL when markets go crazy, but this link is too rigid for all crypto under bitcoin even when it’s relatively quiet.
I don’t understand an exact reason behind this, but I believe it’s related to how crypto exchanges work internally.
The biggest (afaik) of these ETH contract-based coins, USDT, is owned by a single company, which is partially backing it by their BTC and it maintains a list of blocked tokens, which is by design. Of course small money is relatively safe, but for large capitals it’s just a joke.
Any smart contract with a centralized oracle entity has all of the downsides of centralized money. These are much more convenient as a medium to crypto, but that’s it.