(And this is while benefits continue to be cut back: $150/month for many prescriptions, etc.)
As others have written, there's no single reason. But, I think there's a major unintended structural problem: under US law, insurance companies are required to pay a high percentage (like ~90%) of their premiums out to service providers. The intent is to cut administrative overhead.
The effect: it's very hard for an insurance company to invest in technology or administrative improvements. So, the status quo persists.
WORSE, the admin overhead is pushed onto the providers, so the overhead cost gets hidden. It's not uncommon for a family medicine general practitioner to have a back office of 3-5 people dealing with billing and insurance paperwork.