Because the choice of others to not vaccinate affects everyone. I know this is not the case with a lot of vaxxers. But the thing is, it does concern me, when people don't vax, it *is* my business. Because like you, I believe in herd immunity. When people do vax in high numbers, then that is a positive thing for everyone - which is maybe why people don't feel the need to get them to prove their choices.
You just need to look at the measles outbreak in France that is happening right now - 15,000 people so far to see what happens when the vaccination rate falls below a certain number.