It’s a closed circuit. When you’re bleeding out air the pressure in the system drops. When there’s no sufficient pressure the water wont reach the upper floors.
Under normal circumstances the system is bled of air once and then they add more water untill the optimal pressure is reached and that’s it. You shouldn’t need to bleed air or add water ever again. The fact that you keep bleeding it indicates that new air is introduced somehow. Usually this means there’s a leak and someone keeps topping it up with fresh water which contains air that then need to be bled out again.
Alternatively the issue could also be non-plumbers like yourself who keep bleeding the radiators on their own and then the maintenance keeps adding more water and wondering why the pressure always drops again. In my part of the world it’s forbidden for the residents to bleed their radiators for this very reason.
Also, the radiator lines are usually steel. The reason they don’t rust is lack of oxygen. Now with constantly more water and thus air being added to the system the corrosion keeps on advancing.