The reason is that private health insurance companies suffer from the Classic Agency Problem.

They want their costs to increase at a sustainable rate so that they can keep growing their Net Income.

Historically, private health insurance companies earn ~ 6% of their total revenue as profit. To justify an increase in their premiums they have to "miss" their payout ratio slightly every year.

Drugs that patients will take for life are a great way off locking in that increased cost in a "controlled" way.