If you carefully examine what you signed, you may discover that you agreed to be responsible for the entire bill. That is consistently what the agreement says for all of our clients. The doctor may agree to accept insurance monies, as a courtesy to you his patient. Or the doctor may agree to bill your insurance for you, as a courtesy to you his patient. But some doctors put it all on the patient - they expect 100% payment from the patient, and it is the patient's responsibility to bill his own insurance company for whatever monies they may send to the patient. Perhaps you have encountered one of these doctors?
Hint: if your doctor is a participating provider with your insurance carrier, he is bound by his contract with them. If he is not a participating provider, he can require anything from you that your state does not forbid.