I think there are valid questions about the medical efficacy of annual checkups that are heavily weighted by individual patient characteristics such as existing conditions, age, weight, etc.

Seperate from medical efficacy there are a lot of reasons why insurers want you to have an annual visit with a medical provider which has to do with:

1) Assignment of you to a provider. Many, many people are autoassigned a primary care provider by their insurance, they often change this when they actually schedule a visit. Allocation of patients to providers is a very large aspect of manging health plans.

2) Data aggregation and validation of your information. People sign up with unbelievably garbage information and it tends to persist, even on employer provided plans.

3) Baseline medical status such as weight and vitals. Insurers aren't looking at this information directly but it comes through via billing codes for the exact types of exceptional cases insurers want to measure.

These have a huge impact on the actuarial aspects of developing and managing health plans.

PS: Depending on the patient pool, for lots of pools insurers no longer put much economic weight into prevention as it has played out that insurers end up paying for the prevention but patients change insurers so often that they change before the insurer benefits from the effects of the prevention. Incentives are a very messy thing in healthcare between patients, providers and insurers.