Could it be because anti-vaxxers are generally anti-medicine and are therefore not getting tested when they probably should?

Obviously, if you don't believe in Covid, you're going to have way less Covid. Because you don't even believe it exists - so when would you ever admit to having it?

It could also be that the people who get boosters happen to be the most likely to contract Covid, and that's why they're getting the booster. Like if they're immune-compromised, or work at an Airport or something.