Most people don't have the personality for EM. They self select out despite the idealized thoughts they have during pre clinical years.
Plenty of people think too much. And they're brilliant, but their brilliant when they have time to think and would drown with the ER volume of decisions. Being good enough but decisive quickly is better than brilliant but slow. Obviously brilliant and quick takes the cake.
Many people are prestige oriented. There is no prestige in EM. You're the person sifting a pile of junk into junk to be discharged and junk to be admitted and you'll be derided by your specialists no matter what you pick for not picking what they would.
And some people just can't take the vomit, blood, pain seeking, psychogenic illness, and feces that make my day to day life so colorful.
These people will come in enamored and leave absolutely exhausted and overwhelmed, smelling a bit like urine and wondering why the medical team chewed you out for admitting that woman that might have had a stroke.
But there are those of us who put up with it all for the adrenaline and the stories that no one can beat (everyone else's best story came through our ER first.). Working 12-16 shifts a month isn't too bad either.