To add, varroa quickly gains immunity to the pharmaceutical treatment we have, so the same medication cannot be used 2 years in a row. Most popular treatment from late 90s that used to kill 99% of varroa is now completely ineffective.

It was explained to me this is well planned and solved in Czechia. Varroa treatment is refunded my the government, but only one type of medication every 6 months. It's cheaper for beekeepers to use whatever the government gives them for free, than use something else. And the medication is free only for a few weeks, so everyone will use it at the same time.