My mom worked answering phones for a major health insurance company for 8 years and says the same thing. The reason denials work so splendidly for insurers is that people don’t go to the trouble of appealing because they assume it’s a lost cause. If you’re tenacious, your appeal can (and often will) be accepted.
She also says that one time when her supervisor was about to be audited regarding the number of cases pending appeal, she caught him throwing away four filing cabinets full of appeals into the dumpster out back. It has always haunted her how many people those papers represented who never knew why their cases ended up lost or denied.
It’s worth the hassle to try again and again.