Loans are an important concept, that's true. Do blockchains inherently prevent loans? I'm not sure, I think Ethereum smart contracts enable the concept of loans. The 3rd party here is the contract itself, as I understand it.
Blockchain "loans" are fully collateralised and sometimes even require upto 4x the collateral than the loan itself. This is not useful in the vast majority of cases
If I lend you 1 ETH, I have no means to make you pay me back. Smart contracts can't do that either. A smart contract cannot seize 1 ETH from your wallet and send it back to my wallet. Blockchains are designed specifically to prevent that.
That's not true, you can definitely do this with a smart contract blockchain like Ethereum, and there are plenty of loan protocols already that do this in production with $ billions in assets transacted.
No one can steal your ETH from your wallet, but they can liquidate your staked collateral held by the smart contract.