UST actually burns/mints LUNA to try and keep the balance in check. Everything to run the currency is within the same system.
DAI requires 150% collateral in a currency completely outside the system that it can't burn or mint.
It is a big difference.
edit: UST is going to be collateralized in the future if it survives this https://twitter.com/stablekwon/status/1524331190995484672