Does what's considered normal matter for safety? Shouldnt it be based on what the science says?

So? That's an ad populum fallacy. Remember when plenty of places thought smoking cigarettes everywhere was normal?

In addition, laws often contain compromises for feasibility. I expect many jurisdictions permit hands-free conversations partly because it's difficult to prohibit them.

If you want to argue that phone-conversations while driving are safe compared to quiet driving, you should be pointing to experiments rather than popular belief.

Surely everyone is aware that driving while speaking on the phone is distracting. My partner occasionally tries to have an inane conversation about nothing while driving and I always cut it short.

I think that the main problem is if they were in the car with you then they'd be able to modulate the conversation in line with parts of the drive which are more or less demanding. It's much harder to do this successfully on the phone.

Nothing about operating a vehicle is safe. I think it's unfair to judge someone morally for doing an activity that is common practice.