Liability, and hassle. They have a few repairs they do regularly. Anything else is a risk and they don't need the money that badly. And they can't be 90% confident they'll get it right, since they don't do it regularly.
They also can’t charge if they can’t fix it. And they don’t know the extent of the damage beforehand. So they could waste an hour of labor before finding its unfixable.
In that time they could have done 3 iPhone battery replacements.
In that time they could have done 3 iPhone battery replacements.