A lot of it is based on the expectation of having loans paid back with interest. That's why banks have loan officers and all that actually consider the risk to the bank of lending the money, and why you have to apply for a loan instead of just getting one.
If I have a thousand dollars, and I have three people asking to borrow $500 each, I have to consider each of them. I know they're all good for it, so I can give them all $500 by "borrowing" money that James also gave to me to hold for him. A month later, they all pay me back $600 each, so I return the money I "borrowed" from James and have now also made $300 for myself by using it.
Now, one friend has asked for $2000. I know he's been one to not pay back very reliably, so I can't in good faith "borrow" $700 from James along with my $1300 and lend it to him, so I deny.