You are totally mistaken. You absolutely do NOT have to move out of densely populated areas to do private practice. There are plenty of opportunities even in the tightest markets like Chicago, Manhattan, San Diego, etc. The percentage of residents going into some form of private practice varies from 70-80% each year.
From a pateint perspective, you can only go where your insurance lets you. Many insurance companies have contracts with private groups in the area. Unless you are independently wealthy and able to shell out $100K to be treated at a big academic center, you will really have a difficult time convincing your insurance company to let you go "out of network." Besides, many patients would much rather go to a local Rad Onc facility if the are going to be treated for 5+ weeks and not temporarily move somewhere else. Finally, you don't need to go to MDACC to be treated for bread & butter stuff.