How is it legal for a dental company to approve a procedure and then deny it after we started the process?

the dentist office called me saying the insurance approved my upper denture so they scheduled me and I had teeth extracted. When I went back in 2 weeks to have the other teeth extracted and have my temporary placed Was told the insurance company denied the claim and they couldn't do anything. Completely devastated that I have half my teeth missing from my upper palate. I would have never started this process if it was going to be denied because now I'm walking around with missing teeth and it's very difficult. I just don't understand how they can do that . Now I have to figure out how I can get the money and pay out of pocket so I can finish this procedure because like I've said this is been very difficult and I'm a paramedic and I have to deal with patients all the time it's embarrassing