Online lenders have automated much of the underwriting process so the main cost is actually CAC rather then staff overhead.
Loans are generally designed to be recoverable (requiring personal guarantees, assets, etc) so even in case of defaults lenders will recover most of their money (recoverability is also the reason it's normally capped at 10%-20%)
However with fraud the chance of recovery is essentially zero, so fraud is typically what blows up lending books.
With small businesses it's often hard to distinguish legitimate business from fraud and that's the real bottleneck.