The smaller you make the loan, the more pointless it becomes for more and more of the population. For clarity, consider the extreme case: very few people would consider taking a loan of a penny, however many people would consider taking a load of $10,000. A penny is a negligible fraction of your earning power, so it makes more sense to wait 2 seconds at work than to spend 3 seconds taking out a penny loan (and I'm being generous to modern tech; it will take you longer than 3 seconds to take out a loan).

Credit cards acknowledge this issue by construction. They assume that there's some overhead (convenience, time, effort, etc.) with paying back a loan, so what they effectively do is allow you to batch transactions together and pay them all back at once. That way, you don't have to log in every time you make a purchase to "refill". Of course, if this "refill" has no overhead cost, then it wouldn't matter.