The general public doesn’t have enough information to make informed decisions when it comes to healthcare. This alone completely removes the usual market forces from providing any benefit when it comes to healthcare.
Cancer treatments don’t inherently cost that much money, the systems to ensure people are actually getting useful treatments are expensive. You can’t trust companies selling cures. You can’t trust every doctor when they have financial incentives to offer treatments. Insurance companies are in an adversarial relationship with providing treatments, which doesn’t result in efficient supervision here. Lawsuits offer some protection, but at extreme cost to everyone involved. Etc etc.
The net result of all these poor incentives is single payer systems end up being way more efficient, resulting in people living longer and spending less on healthcare.