and I say the main reason is because they are called "altcoins". they are alternative to bitcoin, which makes them not be stand alone projects.
and the other main reason is that these altcoins are purely for speculation purposes there are no other usages for them that makes the market move as one.
almost all the altcoins are forked from bitcoin one way or another. and the rest have at least something in common with bitcoin. the same reason that may cause a fall to 0 in bitcoin will cause the drop to 0 in all the altcoins too!
I never used USDT before
it is shady as hell and if anything happened to bitfinex or the company it will drop to 0 in less than 5 minutes.
Yes tether is centralized but if you play with a strategy , then tether is a very useful coins. For example if BTC went to 20000$ and you sold out and have tether. And later btc went to 10000$ then you will be having 20000$ tether as tether is almost stable for its price and you can buy 2 BTC's then.