Autorotation is a thing.
Helicopters generally fly fairly low which precludes time for proper canopy deployment and determining a safe LZ.
Risk from the blades/falling debris.
People generally need to be trained to parachute. If you're unstable in free fall, as someone untrained would be, you're prone to improper canopy deployment which drastically increases the likelihood of a malfunction and negates the entire point. There's a good reason people go through AFF, etc.
Seen a few comments on this. Parachutes cannot just be thrown on in five seconds, and certainly not in an emergency situation where you're in a close space with others, buckled in, with the chopper going apeshit around you, and that's granted you'd even have time to react. Rigs are quite cumbersome to get into and strap tight. Even if you were in a chopper already fully rigged, in an emergency situation you'd more than likely be dealing with g forces and extreme disorientation and chaos. It's just not going to happen. Even a properly trained and experienced parachutist would be taxed towards surviving.