Very important subject and insufficiently examined. This is THE outstanding phenomenon of the process of decadence of this civilization. I suspect that it will ultimately be explained by Schumpeters theories, I.e., that rent seeking, cronyism, regulation, and paralysis through bureaucracy will cripple economic growth. More at some periods than others, but the general path is the same.
Some years ago (the 1990s) I was studying the impact of government regulation on street vending. There were considerable efforts by govt. and big business in places like New York, Philadelphia and Washington, DC to make it much harder for small independent steet vendors to operate. In DC in years prior to that there had been something like 7,000 independent vendors. People also drove trucks with fruit and vegetables around the poorer neighborhoods where supermarkets were not available. Now about 85% of that business has been destroyed, and black unemployment is extremely high.