Generally not the insurance companies. While they might have large profits in gross terms, their margins are somewhat constrained by the combination of competition and Medical Loss Ratio (MLR, the amount of revenue spent on care). MLR for large group policies is 85% (of every $100 in premiums paid, they must pay out at least $85 in care).

None of which should be construed that they don't contribute to the problem. They just aren't the primary villain.