Many reasons one being it damages your blade when it kicks in! And it can be triggered by wet wood or metal in wood not just a human finger.
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I purchased a SawStop about 2 years ago and my feeling was even if I Accidently kicked it a few times and I'm out a $100 blade and a $69 cartridge (how much is a surgery or amputation) each time its worth not losing my finger. I cut a piece of wood that was 18% moisture content and the saw did not trip. To me it is a cheap insurance policy that I will never lose the ability to do woodworking because of a split second accident.
As far as the inventor seeking mandatory SawStop placement I agree it should not be mandated, however I still think owning one is a good decision.