Holly - because the person standing next to you may not be able to be vaccinated, and the mother picking up at the door may be in the middle of immune-suppressing chemo, so that contracting a virus spread by the healthy-but-infected child could be fatal.
There's an argument for 'why should I put other people's health ahead of that of my child' which is fair enough, if a little 'I'm alright, Jack', but the flip of that argument is 'why should your child's health be any more important than mine?'
It's a minefield.
The easy way to think of this is to imagine a viral outbreak as a flood and immune people - whether immune naturally or created so - as flood barriers. The more barriers, the less the flood damage. If the gaps are small enough,the virus can't get out and spread and grow, and eventually, hopefully it dies altogether.
But, not enough barriers, and there's water everywhere, which is bad for everyone - and that's what's starting happen with some virii and the uptake of immunization. Something will have to be done, because the NHS won't cope with the return of generalised measles etc, but I'm not nearly smart enough to work out what.