Because a) the risk from covid, while small, is still greater than the risk from the vaccines and b) unvaccinated people are more likely to be infected and become a vector for the virus, infecting other people.
I'd add to this the fact that it was especially true earlier in the pandemic, and at that time COVID was significantly more likely to be fatal. Even if someone rejects the idea of getting a fourth dose of the vaccine (rightly or wrongly), it's possible to think that the first two doses were an entirely reasonable one-time intervention into an ongoing pandemic that saved many thousands of lives.
I've read conflicting accounts of whether that's true or false, but even if it is - what difference does it make? We're not going to reach heard immunity here. Everybody's going to get it, several times throughout their lives, no matter what we do at this point.