Why they are not completely trustless ?
They are DEX and use SmartContracts ( Open Source )
See
honglu69's post above.
In other words:
"Traditional" atomic swaps make it completely impossible to steal funds, at least if you aren't able to 51% attack one of both blockchains. The reason ist that if the secret of the HTLC/PTLC is revealed, only then both parties can move the coins they have traded. If not, both can't. There's thus a "direct connection" between both transactions.
In Thorchain and other similar concepts there isn't such a direct connection. There are different classes of special nodes which are in charge of the exchange trades. These nodes get money if they carry out the trades orderly and if not, they are punished. This (the incentives) is what is ensured by the smart contracts, on ThorChain, not the cross-chain exchange itself.
I think the general safety of ThorChain can be compared with that of an algorithmic stablecoin (e.g. Dai). This means, that while the nodes carrying out the swaps in almost all cases will do well if they play fair, it isn't guaranteed that there's no strategy for them to profit stealing funds.
Basically, atomic swaps ensure the exchange with the consensus mechanisms of the participating blockchains. Thorchain ensures the exchange with its own consensus mechanism, which may be weaker than Bitcoin's and other big coins' consensus.