Why do people still get whooping cough even though they were immunized?

Unlikely but possibl: There is no vaccine that is 100% guarantee you will never get the disease effective. Also, if your body has certain types of immune deficiencies, some vaccines might "not take" for you and you won't build up an immunity for that disease. So while the technical answer to your question is yes, it is very rare to get disease after immunization. However, protection may wear off and you need boosters!

Waning immunity: Those vaccinated as kids before the early 90's received the whole cell vaccine that was never given after 5 and many began to lose immunity by their teens. After the mid 90's we began using the synthetic vaccine with less side effects & we were able to give it to pre-teens.However, any vaccinated older adult is likely vulnerable due to waning immunity.