Because there are always people who for different reasons cannot. Either because they're too young, or immuno-suppressed, or because they react adversely to vaccines. We know that. We know that not 100% of the population can get vaccines, but the only way to stop the weakest among us getting sick is to close off as many vectors of contagion as possible.

As you rightly say, meowmee, since you get sick from the vaccines, your family members who do not need to get vaccines to protect you. In my family, my brother had an adverse reaction to his first vaccine as a baby, so he hasn't had all his vaccines. It is therefore imperative that we around him make sure we get the vaccines we can. He was lucky enough to survive without complications the mumps as a child - adult men who get the mumps sometimes get sterile, while some kids become brain damaged, deaf, or suffer from pancreatitis (which heightens the risk of diabetes type 3c and pancreatic cancer down the line.)

Right now, all our family have had the flu vaccine since a close family member is doing chemotherapy. I am fully aware that if I get even the sniffles just before Christmas, I'll be staying home and not celebrating with my family who will be with my sick family member. That's how dangerous it is for them to get so much as a cold right now. A viral infection could kill them - a childhood disease most likely will. They have to restrict their contact with strangers, and their kids aren't allowed to bring friends home because they live in a trendy area with lots of anti-vaxxers who think having a child with autism is worse than a dead kid, or killing others.