If you have money in another form doesnt mean that you want to sell this. For example a steam account. You might want to keep it. Though you still can give it as collateral to an escrow.
And why should someone accept 50% risk? The scammer could have planned to scam an amount 50% of the taken loan. Then you gave him 100% of what he wanted that way.
It's not just about scams, its also about accidental defaults. There could be cases where the loaner just simply cant pay the loan back and it defaults on it.
For example if you give out 0.5 BTC for a gambler and he lose it all, but his income is from faucets only and he cannot pay it back at all.
In that case you need to factor that in, every bank or loaning agency does the same, there is mathematical part you cannot ignore when working with money , merely probability & statistics
If many scammers appear suddenly then you got 2 options: raise the interest rates to cover the losses, or implement stricter rules for giving out loans.