Flying to another country implies you have the money to front a ticket to another country, that you've had the foresight to get a passport to go to mexico (I guarantee if you asked many poor people if they had one, they'd say no) and that you can actually take time off to do said medical tourism.

For a lot of people? A few days off means they lose their job and everything else. So they work through the pain or the illness until it becomes a severe medical emergency.