People getting vaccinated are taking "one for the team" and at the same time "looking out for number one". That's the beauty of it, really: aligned incentives.
All vaccinations have some level of risk. If you are surrounded by unvaccinated, your risk of not getting the vaccine outweighs the risk of catching the disease from the herd. In this instance you have aligned incentives.
However, if you are in a entirely vaccinated population, your risk profile changes, and it is now possible for your incentives to become unaligned with "the team", or at least more blurry.
Unfortunately the average antivaxer's mental model is probably not so well defined that they realise is what they have been doing. I would not be surprised if the majority of them don't realise/agree that they've been selfishly coasting on the coat-tails of the crowd with other vaccines/diseases, and therefore they have trouble realising their currently "successful" strategy is not going to work so well this time round.
I got vaccinated to protect myself. I am aware that even with 100% vaccination rates, sooner or later i will get in contact with covid (and/or i probably already have), and the vaccine is protecting only me. I am not afraid of the unvaccinated, and I don't care if they choose a greater risk, it's their life, and their choice.