Because the vast majority of people in a theme park are of good to excellent health. Cities have homeless, poor elderly and many other that are just to ill to go to a theme park and walk 10 miles a day. The likelihood of a need for EMS is very low in WDW. Not impossible but low. As I stated before most would be falls or fatigue. It isn't the same. Even in cities with many EMS units there have been times when the wait was almost or definitely too long.
The land space in WDW is often compared with Manhattan, NY. 40+square miles. The population of Manhattan is 1.6 million and that's just at night, during the day when people go into the city to work it probably doubles. What Disney has, based on first the seriousness of the problem and the numbers of people and issues over all, is adequate. Can they add a few more people to the staffing, probably, but that is true with any service such as this. This is just an over reaction to a statement made by a union leader to apply leverage. It is not a real problem anymore than it is in the real world.