What do you do if your own adult kids are harassing and bullying you? Where can you go for help? Why there are no agencies that help parents in such situations?

There are people and organizations that can help in these sorts of situations. If you are an elderly adult and are being abused by family members, that may constitute elder abuse and can be reported to and investigated by your state's Adult Protective Services (like CPS but for older adults). Even if this is not elder abuse, in any case, you can look for a child and family services agency or community mental health center in your area. These agencies have social workers and family therapists who are trained to help families navigate situations like this.