Because it’s a massive franchise. When you go into a Starbucks they want it to taste the same no matter where you get one. People are creatures of habit and it’s easier to get someone to get the same thing than it is to try out local shops wherever they go because it might be bad or too different from what they normally prefer. It’s one of the reasons fast food chains are / were very popular in the US. It’s not about the quality, it’s about the consistency
Actually it’s worse than that, if you have enough unvaccinated people in an area you’ll increase the viral load received by the local population. Vaccines raise immunity significantly, but don’t make you fully immune. If you experience enough of a viral load despite being vaccinated you can still get sick. This is how outbreaks occur and why we’re seeing them in low vaccination communities. These viruses then spreads to others that shouldn’t normally get the virus. So in short it harms everyone including those vaccinated.