Why does passive immunity usually not last for very long?

Answer 1

When you have immunity, your body has antibodies against a foreign organisms (i.e. bacteria and virus) .

Usually this is achieved by exposure to these organisms.

That is: 1. you get an infection (or vaccination with inactive parts of a bacteria or virus), 2.your body defeats the invader, 3. some of the cells in your body "remember" which antibodies they produced , 4. next time you are exposed to the same foreign organism, these cells quickly produce antibodies so you won´t get ill (i.e. you are immune)

When you have passive immunity, it usually is due to a vaccination with antibodies. Your body hasn´t produced the antibodies on its own. So once the antibodies become too old and inactive, you are not immune any longer, and when you encounter the foreign organism again, it will be like a novel infection, because it has no memory cells, which can respond quickly.