Well Black Betty is meant to be only as durable as Ram Jam herself is to kind of balance out the fact that not only does she possess intelligence, autonomy, and can equip her Stand user to fight while remaining combat-effective herself, but also has guns that can fire magic bullets. Having her be so defenseless means if either Ram Jam or Black Betty take a hit, Ram Jam is going to feel it.

If she could take a hit, she'd be really OP.