that's because you are using arbeth on the blockchain Arbitrum so whatever you do with USDT you must have a remaining arbitrum fee for the transaction to continue. Likewise with ETH. remember the Ethereum and Arbitrum blockchains are different.
its L2 of ethereum so basically its the same mechanism and fee in arbitrum is also using ethereum so there's no difference really the only thing you need to do is just bridge the ethereum so that it can be used in the blockchain of arbitrum,
it seems OP mistaking arb to be used as a fee in the blockchain while its actually ethereum, arb is just ERC20 token to represents the blockchain but the fee actually used ethereum since its L2 basically means side chain of ethereum, so of course the transaction not gonna went through.
once OP can deposit some ethereum that already bridged to arbitrum, then OP can execute the transaction.
why is that, because its standalone blockchain its just that the data of transactions in arbitrum getting submitted back to ethereum because its needed as a layer 2.