Last Uber drive that I talked to that did this on the side was in the area for some construction job that required him to be on site some days of the week, others didn't. In those days when they didn't call on him he did Uber for some money. The alternative was to not do anything and not get that money. There are definitely costs (hidden or not, really depends on how deep you are going to look for them) but I can also see how given the alternative of not doing anything they can still help. It's not like the Uber driver doesn't have to pay for insurance, registration and other fixed car costs (food, rent) anyway, whether they Uber or not.