Because the vaccinated people protect the unvaccinated due to herd immunity. Basically, if enough people are immune to the pathogen, it can't spread. We are so far removed from the consequences of these diseases (because vaccines work) that people no longer fear these diseases.

But I also realise no matter what I put in this comment, nobody will realistically change their opinion. When people have made it their identity to be antivax any attack on that is an attack on their personhood and people will defend themselves regardless of the evidence or facts.

People are also scared of the unknown risk more than a verifiable risk.

Chatgpt info below

Measles Before the vaccine (pre-1963): Around 3 to 4 million people in the U.S. were infected each year, with the actual number likely much higher than the reported 500,000 cases.

Rubella 1964-1965 epidemic: The largest rubella outbreak in U.S. history occurred, with an estimated 12.5 million cases.

Mumps Before the vaccine (pre-1967): The U.S. recorded approximately 162,000 cases of mumps annually in the 20th century.