Market lenders aren't banks. They cannot use fractional reserve banking. If they take in $100 they loan out $100. A bank takes in $100 having loaned out $10K to satisfy reserve requirements 10% (likely less but keeping it simple). So if most lending moved to market lenders we would see a collapse in the money supply.
If the bank's reserve requirement is 10%, wouldn't it only be able to lend out $90 of the $100 it had taken in deposits?
Here's a chart that shows the expansion potential of money at various reserve requiements: http://en.wikipedia.org/wiki/Fractional_reserve_banking#medi...
This also shows how banks make so much money, and why there are both laws and services designed to encourage people to keep their money in banks. A $100 deposit generates slightly less than $1K in loans. At only 5% interest, the banks will realize $50 in interest per year on the ~$1K in loans enabled by that single $100 deposit.