No, it’s because rearers will flip over on you. Maybe the ones you’ve worked with haven’t (yet) but it is not uncommon. Getting flipped on is nearly certain to land you in the hospital, maybe even in a casket.
I’m not trying to downplay the seriousness of rearing. But I do genuinely feel it is overstated on this forum. The overwhelming majority of rider deaths are caused by head and neck injuries, not the thoracic or abdominal injuries you would expect from being crushed. It’s not the hill I want to die on, if there’s research or something out there showing that a rear is appreciably more dangerous than other vices, I would let it go.