That’s probably a factor (insurers make enough payments to hospitals they can demand lower prices and get them while out of pocket people usually don’t have much power to negotiate), but also this article makes it sound like $1400 is just the base price,

The report estimates the average ambulance ride in the United States costs $1,383.44, more than double Australia’s $622 average. The gap widens further with mileage fees: Americans pay about $19.49 per mile, while most other countries charge little to nothing based on distance traveled.