A few flight attendants disclosed that they were told not to lift any luggage during their training for their specific airline. When passengers board the plane and need to stow their suitcases in the overhead compartments, they are supposed to do it alone. If a flight attendant does it for them and gets hurt in the process, the company will not cover any compensation because it's intentionally not in their job description.
However, if a passenger is elderly or can clearly not lift their bag, most flight attendants will help them anyway. Though it is highly discouraged, so they try to keep it to a minimum.