The fourth word on Tether's wikipedia page is "controversial". Nobody is under the impression that it's fully collateralized. Would you care to make the case that a different, trusted stablecoin isn't fully collateralized?
DAI specifically is collateralized with ETH using smart contracts - it's over 100% collateralization is incontrovertible.
Sure, it's collateralized by ethereum - the problem with that of course is that ethereum is a completely manipulated price driven up by billions in falsely collateralized tether.
So effectively, you've linked DAI to the value of a coin that is set by tether. That being the case, the actual value of the collateral is in question.
DAI specifically is collateralized with ETH using smart contracts - it's over 100% collateralization is incontrovertible.