It's really simple. Past experience has shown that pilots do stupid things for money. We take risks we shouldn't when our paycheck is on the line. It used to be that I didn't need a commercial license to fly for money, but after seeing too many people get injured and die because of risky behavior (weather, fuel, maintenance, etc...) the FAA saw that regulations about commercial flight were necessary.
Imagine there's a blizzard outside. Think about your motivations to drive yourself to a friend's house, vs. your motivations to drive an uber passenger to his friend's house when the client is paying a 10.0x surcharge. And you really need the money because you're short on rent. Think you're more likely to take the risk when there's money on the line? Of course you are. We all are. But in aviation, you're always one hiccup away from an accident so the consequences of chasing money are much worse.
And that's why $50 makes all the difference.