It isn't all about getting somebody TO the hospital but getting them INTO the hospital/ED/ER. EMS in an ambulance who are alerting a hospital of an MI enroute will get their attention, a walk-in will have to wait unless there are obvious signs.

Calling 911 will normally get LEO on scene that know CPR and can do radio communications. A lot of dispatchers are EMDs (emergency medical dispatchers) that can start helping immediately. You may have off duty EMTs nearby that are scanning the radio. Finding a fixed target it much easier than finding a moving target (white car headed towards hospital), you are on your own if you get stuck in traffic. Statistically, 911/EMS is the best outcome. I agree with another commenter, exceptions do exist.