I thought that the reason why you don't catch measles, polio, etc. is partly due to the fact that you got the vaccine, but also largely because everybody else got the vaccine, too. This is called "herd immunity". Every time you get exposed to a virus, you roll the dice. Getting a vaccine gives you a hefty bonus to that dice roll, but you still want to roll those dice as little as possible, because no vaccine is 100% effective.

One way that influenza and COVID are different is that they mutate like crazy, which is why you have to keep getting updated booster shots every year. With COVID, not only do we have a bunch of different strains flying around, but we also have people who refuse to get the vaccine, wear a mask, stay home when they're sick, and so on, and COVID is super-contagious. So, while getting a vaccine will help load the dice in your favor, the odds of you never getting it are extremely low, even if you do everything right. But that dice bonus will still lower the odds of you getting sick, and it also lowers the odds of severe illness if you do get sick.