Most of them are bad at math because they ignore a lot of variables like wear and tear on their vehicles and the fact that their car insurance doesn't cover them using their private vehicle for commercial purposes. It's one of those things that profitable in the short-term but only if you ignore long term costs. Granted some people might have somewhat legitimate reasons for ignoring those costs or may have made some cost benefit analysis in which it makes sense for them, but overall it tends to be a net negative if you actually include all of the costs.