A new street food vendor only competes with other food vendors in the area. A new game developer competes globally, against both established studios and hobbyists happy to work for free because it's fun. You have people who want to make games so badly they'll fight to get in, knowing fully it's low pay and long hours.
Any way you look at it, the reason game developer salaries are relatively low is because supply for the jobs is higher than the demand.
It's a bit silly to compare the two because the economics are so different (local, non-scalable product vs. global product with zero marginal cost), but I stand by my assertion that there's no inherent reason why game developers should earn more, on average, than food vendors.