300 Blackout exists and people use it. Why?
I have no idea, but the trend for short barreled big heavy slow bullets "sniper" technology is partially to blame for 223 runners.
Shot placement. Everybody now thinks they are a long range sniper with their budget milspec rifles. Ask any Army vet, they'll brag about how great a shot they were at 500 meters with ball ammo and iron sights. That's just not good enough for ethical hunting.
A lot of hunters don't have field marksmanship skills to hit a coyote in the actual vitals effectively. Runners are almost always gut shots, or clipping the rear of the lungs or liver. Argue all you want, runners are almost always due to poor shot placement.
Heavy long range bullets play a role too. The trend is heavier slower bullets with higher BCs which are likely to pass right on thru with less fragmentation/expansion then other styles.
Optics. A lot of guys are using crappy optics with flip up magnifiers, dot scopes, thermals that don't hold zero or are hard to zero, and high sighs on AR15s that create different zero's/holdovers then they're used to on deer rifles. Add in all these new technologies like BDC circles and hash marks and thermal etc and many don't practice enough with their gear to be effective in the field.
There was even a popular scope couple years back that lacks a center reticle, it just has a circle instead. This does not lead to accurate shots, sorry.
Short barrels/reduced velocity. Lots of AR15 hunters out there using 14.5" and 16" barrels with drastic velocity reduction compared to the standard 22-26" bolt action barrel of yesteryear. Even on bolt actions these days everybody wants to chop their barrels so they can add a silencer. Put the bullet where it matters and it should still work, but the reduced velocities also reduce the margin of error, a marginal shot may have been more effective if it was 300fps faster.